Extra specs for String#append_as_bytes #1879

Job	Run time
RuboCop	29s
specs (ubuntu, 3.0.6)	1m 32s
specs (ubuntu, 3.1.4)	1m 34s
specs (ubuntu, 3.2.2)	1m 25s
specs (ubuntu, 3.3.0)	1m 28s
specs (ubuntu, 3.4.1)	1m 35s
specs (macos, 3.0.6)	1m 23s
specs (macos, 3.1.4)	1m 21s
specs (macos, 3.2.2)	1m 11s
specs (macos, 3.3.0)	1m 10s
specs (macos, 3.4.1)	1m 11s
specs (windows, 3.0.6)	4m 46s
specs (windows, 3.1.4)	5m 21s
specs (windows, 3.2.2)	4m 58s
specs (windows, 3.3.0)	4m 34s
specs (windows, 3.4.1)	4m 58s
specs (ubuntu, 3.3.0, --enable-frozen-string-literal)	1m 23s
	40m 19s

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