Add an option for WriteNode to keep attribute namespaces

* Adds an optional parameter to WriteNode to specify what attribute
  namespaces should be retained
* Supports a FlexBridge fix https://jira.sil.org/browse/LT-21388
  where xml:space was being saved as 'space' which on a further
  write by a normal xml serializer would then drop spaces

+semver:minor

SIL.Core.Tests/Xml/XmlUtilsTests.cs

@@ -176,6 +176,36 @@ public void WriteNode_DoesNotIndentChildWhenSuppressed()
 			}
 			Assert.That(output.ToString(), Is.EqualTo(expectedOutput));
 		}
+
+		[Test]
+		public void WriteNode_PreserveNamespacesArePreserved()
+		{
+			string input = @"<text><span class='bold' xml:space='preserve'> </span></text>";
+			string expectedOutput =
+				"<?xml version=\"1.0\" encoding=\"utf-16\"?>\r\n"
+				+ "<root>\r\n"
+				+ "	<text>\r\n"
+				+ "		<span\r\n"
+				+ "			class=\"bold\"\r\n"
+				+ "			 xml:space=\"preserve\"> </span>\r\n"
+				+ "	</text>\r\n"
+				+ "</root>";
+			var output = new StringBuilder();
+			var preserveNamespace = new HashSet<string>
+			{
+			   "xml"
+			};
+			using (var writer = XmlWriter.Create(output, CanonicalXmlSettings.CreateXmlWriterSettings()))
+			{
+				writer.WriteStartDocument();
+				writer.WriteStartElement("root");
+				XmlUtils.WriteNode(writer, input, new HashSet<string>(), preserveNamespace);
+				writer.WriteEndElement();
+				writer.WriteEndDocument();
+			}
+			Assert.That(output.ToString(), Is.EqualTo(expectedOutput));
+		}
+
 		/// <summary>
 		/// This verifies that suppressing pretty-printing of children works for spans nested in spans nested in text.
 		/// </summary>
@@ -191,8 +221,10 @@ public void WriteNode_DoesNotIndentChildWhenTwoLevelsSuppressed()
 				+ "				class=\"italic\">bit</span>bt</span></text>\r\n"
 				+ "</root>";
 			var output = new StringBuilder();
-			var suppressIndentingChildren = new HashSet<string>();
-			suppressIndentingChildren.Add("text");
+			var suppressIndentingChildren = new HashSet<string>
+			{
+			   "text"
+			};
 			using (var writer = XmlWriter.Create(output, CanonicalXmlSettings.CreateXmlWriterSettings()))
 			{
 				writer.WriteStartDocument();

SIL.Core/Xml/XmlUtils.cs

@@ -970,10 +970,11 @@ public static string GetTitleOfHtml(XmlDocument dom, string defaultIfMissing)
 		/// <param name="writer"></param>
 		/// <param name="dataToWrite"></param>
 		/// <param name="suppressIndentingChildren"></param>
-		public static void WriteNode(XmlWriter writer, string dataToWrite, HashSet<string> suppressIndentingChildren)
+		/// <param name="preserveNamespaces">a set of namespaces to preserve when writing out elements</param>
+		public static void WriteNode(XmlWriter writer, string dataToWrite, HashSet<string> suppressIndentingChildren, HashSet<string> preserveNamespaces = null)
 		{
 			XElement element = XDocument.Parse(dataToWrite).Root;
-			WriteNode(writer, element, suppressIndentingChildren);
+			WriteNode(writer, element, suppressIndentingChildren, preserveNamespaces);
 		}
 
 		/// <summary>
@@ -984,11 +985,12 @@ public static void WriteNode(XmlWriter writer, string dataToWrite, HashSet<strin
 		/// <param name="writer"></param>
 		/// <param name="dataToWrite"></param>
 		/// <param name="suppressIndentingChildren"></param>
-		public static void WriteNode(XmlWriter writer, XElement dataToWrite, HashSet<string> suppressIndentingChildren)
+		/// <param name="preserveNamespaces">a set of namespaces to preserve when writing out elements</param>
+		public static void WriteNode(XmlWriter writer, XElement dataToWrite, HashSet<string> suppressIndentingChildren, HashSet<string> preserveNamespaces = null)
 		{
 			if (dataToWrite == null)
 				return;
-			WriteElementTo(writer, dataToWrite, suppressIndentingChildren);
+			WriteElementTo(writer, dataToWrite, suppressIndentingChildren, preserveNamespaces);
 		}
 
 		/// <summary>
@@ -997,11 +999,18 @@ public static void WriteNode(XmlWriter writer, XElement dataToWrite, HashSet<str
 		/// <param name="writer"></param>
 		/// <param name="element"></param>
 		/// <param name="suppressIndentingChildren"></param>
-		private static void WriteElementTo(XmlWriter writer, XElement element, HashSet<string> suppressIndentingChildren)
+		/// <param name="preserveNamespaces">a set of namespaces to preserve when writing out elements</param>
+		private static void WriteElementTo(XmlWriter writer, XElement element, HashSet<string> suppressIndentingChildren, HashSet<string> preserveNamespaces = null)
 		{
 			writer.WriteStartElement(element.Name.LocalName);
 			foreach (var attr in element.Attributes())
-				writer.WriteAttributeString(attr.Name.LocalName, attr.Value);
+			{
+				var ns = attr.Name.NamespaceName;
+				if(preserveNamespaces != null && preserveNamespaces.Contains(ns))
+					writer.WriteAttributeString(attr.Name.LocalName, attr.Name.NamespaceName, attr.Value);
+				else
+					writer.WriteAttributeString(attr.Name.LocalName, attr.Value);
+			}
 			// The writer automatically suppresses indenting children for any element that it detects has text children.
 			// However, it won't do this for the first child if that is an element, even if it later encounters text children.
 			// Also, there may be a parent where text including white space is significant, yet it is possible for the