[WIP] v2.x - Nokogiri Refactor Part 6 - Conversion of SignedDocument and final removal of REXML 🎉 #578

Job	Run time
rubocop	10s
Unit test (ubuntu-20.04, 3)	1m 41s
Unit test (ubuntu-20.04, 3.4)	1m 36s
Unit test (ubuntu-20.04, 3.1)	1m 36s
Unit test (ubuntu-20.04, 3.3)	1m 35s
Unit test (windows-latest, 3.1)	3m 14s
Unit test (ubuntu-20.04, jruby-9.4)	2m 17s
Unit test (macos-latest, 3)	1m 28s
Unit test (windows-latest, 3.2)	3m 12s
Unit test (macos-latest, 3.2)	1m 24s
Unit test (macos-latest, 3.3)	1m 24s
Unit test (windows-latest, 3)	3m 14s
Unit test (ubuntu-20.04, 3.2)	1m 43s
Unit test (macos-latest, jruby-9.4)	1m 59s
Unit test (windows-latest, 3.4)	5m 5s
Unit test (ubuntu-20.04, truffleruby)	3m 24s
Unit test (macos-latest, 3.1)	1m 27s
Unit test (windows-latest, 3.3)	3m 30s
Unit test (macos-latest, truffleruby)	2m 24s
Unit test (macos-latest, 3.4)	1m 28s
finish	-1s
	43m 50s

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