Support frozen strings with REXML #395

Job	Run time
Unit test (ubuntu-20.04, 2.1)	47s
Unit test (ubuntu-20.04, 2.2)	1m 10s
Unit test (ubuntu-20.04, 2.3)	1m 4s
Unit test (ubuntu-20.04, 2.4)	1m 3s
Unit test (ubuntu-20.04, 2.5)	31s
Unit test (ubuntu-20.04, 2.6)	30s
Unit test (ubuntu-20.04, 2.7)	29s
Unit test (ubuntu-20.04, 3)	17s
Unit test (ubuntu-20.04, 3.1)	15s
Unit test (ubuntu-20.04, 3.2)	15s
Unit test (ubuntu-20.04, 3.3)	16s
Unit test (ubuntu-20.04, jruby-9.1)	1m 13s
Unit test (ubuntu-20.04, jruby-9.2)	59s
Unit test (ubuntu-20.04, jruby-9.3)	47s
Unit test (ubuntu-20.04, jruby-9.4)	53s
Unit test (ubuntu-20.04, truffleruby)	1m 48s
Unit test (macos-latest, 2.6)	37s
Unit test (macos-latest, 2.7)	36s
Unit test (macos-latest, 3)	29s
Unit test (macos-latest, 3.1)	22s
Unit test (macos-latest, 3.2)	31s
Unit test (macos-latest, 3.3)	19s
Unit test (macos-latest, jruby-9.3)	40s
Unit test (macos-latest, jruby-9.4)	38s
Unit test (macos-latest, truffleruby)	1m 16s
Unit test (windows-latest, 2.2)	1m 35s
Unit test (windows-latest, 2.3)	1m 39s
Unit test (windows-latest, 2.4)	2m 9s
Unit test (windows-latest, 2.5)	2m 40s
Unit test (windows-latest, 2.6)	2m 25s
Unit test (windows-latest, 2.7)	2m 35s
Unit test (windows-latest, 3)	1m 37s
Unit test (windows-latest, 3.1)	1m 8s
Unit test (windows-latest, 3.2)	1m 41s
Unit test (windows-latest, 3.3)	2m 26s
finish	0s
	37m 40s

Provide feedback