Extra specs for String#append_as_bytes #1881

Job	Run time
RuboCop	35s
specs (ubuntu, 3.0.6)	1m 31s
specs (ubuntu, 3.1.4)	1m 37s
specs (ubuntu, 3.2.2)	1m 30s
specs (ubuntu, 3.3.0)	1m 30s
specs (ubuntu, 3.4.1)	1m 33s
specs (macos, 3.0.6)	1m 25s
specs (macos, 3.1.4)	1m 27s
specs (macos, 3.2.2)	1m 38s
specs (macos, 3.3.0)	1m 42s
specs (macos, 3.4.1)	1m 44s
specs (windows, 3.0.6)	4m 49s
specs (windows, 3.1.4)	4m 57s
specs (windows, 3.2.2)	5m 34s
specs (windows, 3.3.0)	5m 34s
specs (windows, 3.4.1)	5m 36s
specs (ubuntu, 3.3.0, --enable-frozen-string-literal)	1m 28s
	44m 10s

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