[WIP] v2.x - Nokogiri Refactor Part 6 - Conversion of SignedDocument and final removal of REXML 🎉 #575

Job	Run time
rubocop	14s
Unit test (ubuntu-20.04, 3.3)	1m 41s
Unit test (ubuntu-20.04, 3.1)	1m 44s
Unit test (macos-latest, 3.1)	1m 28s
Unit test (ubuntu-20.04, 3.2)	1m 43s
Unit test (macos-latest, 3.2)	1m 28s
Unit test (ubuntu-20.04, jruby-9.4)	2m 20s
Unit test (ubuntu-20.04, 3.4)	1m 37s
Unit test (macos-latest, jruby-9.4)	2m 2s
Unit test (macos-latest, 3)	1m 25s
Unit test (ubuntu-20.04, 3)	1m 38s
Unit test (ubuntu-20.04, truffleruby)	3m 39s
Unit test (macos-latest, 3.3)	1m 52s
Unit test (windows-latest, 3)	4m 23s
Unit test (windows-latest, 3.1)	3m 2s
Unit test (macos-latest, 3.4)	1m 38s
Unit test (windows-latest, 3.3)	3m 19s
Unit test (windows-latest, 3.4)	4m 34s
Unit test (macos-latest, truffleruby)	3m 19s
Unit test (windows-latest, 3.2)	3m 15s
finish	-1s
	46m 20s

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