[WIP] v2.x - Nokogiri Refactor Part 6 - Conversion of SignedDocument and final removal of REXML 🎉 #583

Job	Run time
rubocop	13s
Unit test (ubuntu-20.04, 3.1)	1m 42s
Unit test (ubuntu-20.04, 3)	1m 42s
Unit test (ubuntu-20.04, 3.3)	1m 44s
Unit test (ubuntu-20.04, 3.4)	1m 47s
Unit test (ubuntu-20.04, 3.2)	1m 46s
Unit test (macos-latest, 3.2)	1m 34s
Unit test (macos-latest, 3.3)	1m 40s
Unit test (ubuntu-20.04, truffleruby)	3m 30s
Unit test (macos-latest, 3.1)	1m 30s
Unit test (macos-latest, truffleruby)	2m 27s
Unit test (macos-latest, 3.4)	1m 31s
Unit test (macos-latest, 3)	1m 37s
Unit test (ubuntu-20.04, jruby-9.4)	2m 28s
Unit test (windows-latest, 3.1)	3m 0s
Unit test (windows-latest, 3.2)	3m 49s
Unit test (windows-latest, 3.4)	4m 11s
Unit test (macos-latest, jruby-9.4)	2m 12s
Unit test (windows-latest, 3.3)	3m 10s
Unit test (windows-latest, 3)	4m 34s
finish	0s
	46m 7s

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