Visitors example in README doesn't work

[CoderNate]

The visitors example in the README that adds an xsi:nil attribute doesn't work. The "when" function never sees values that are null. In ObjectNode.cs, members with null values are filtered out in the EnumerateMembers function before they can ever be tested by the "when" function of the visitor:

if (Context.Mode.IsSerialize()) members = members
                .Where(x => x.Value.Instance != null && ...

Also, if the 'when' test did pass, the visitor action would fail when trying to create an XAttribute:

var xsiNamespace = XNamespace.Get("http://www.w3.org/2001/XMLSchema-instance");
var succeeds = new XAttribute(xsiNamespace + "nil", "true"); //Succeeds
// The following fails with XmlException "The ':' character, hexadecimal value 0x3A, cannot be included in a name."
var fails = new XAttribute("xsi:nil", "true"); 

[mikeobrien]

Thx for catching that, I'll get it fixed.