Add specs for `cr_newline` and `crlf_newline` options to `String#encode` #1639

Job	Run time
RuboCop	29s
specs (ubuntu, 3.0.6)	1m 29s
specs (ubuntu, 3.1.4)	1m 28s
specs (ubuntu, 3.2.2)	1m 24s
specs (ubuntu, 3.3.0)	1m 24s
specs (macos, 3.0.6)	1m 27s
specs (macos, 3.1.4)	1m 21s
specs (macos, 3.2.2)	1m 27s
specs (macos, 3.3.0)	1m 42s
specs (windows, 3.0.6)	5m 10s
specs (windows, 3.1.4)	6m 26s
specs (windows, 3.2.2)	4m 41s
specs (windows, 3.3.0)	4m 19s
specs (ubuntu, 3.3.0, --enable-frozen-string-literal)	1m 26s
	34m 13s

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