Add spec to test String literals behaviour #1577

Job	Run time
RuboCop	26s
specs (ubuntu, 3.0.6)	1m 29s
specs (ubuntu, 3.1.4)	1m 29s
specs (ubuntu, 3.2.2)	1m 27s
specs (ubuntu, 3.3.0)	1m 27s
specs (macos, 3.0.6)	2m 4s
specs (macos, 3.1.4)	1m 48s
specs (macos, 3.2.2)	2m 15s
specs (macos, 3.3.0)	2m 45s
specs (windows, 3.0.6)	3m 40s
specs (windows, 3.1.4)	4m 6s
specs (windows, 3.2.2)	4m 47s
specs (windows, 3.3.0)	5m 27s
specs (ubuntu, 3.3.0, --enable-frozen-string-literal)	1m 24s
	34m 34s

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