[fix] Encoding.compatible?: check when swapping args #2358

Job	Run time
RuboCop	47s
specs (macos, 3.3.10)	1m 50s
specs (macos, 4.0.1)	2m 3s
specs (ubuntu, 4.0.1, --repeat 2)	2m 16s
specs (ubuntu, 4.0.1, --parser=parse.y)	1m 34s
specs (windows, 4.0.1)	3m 53s
specs (windows, 3.4.8)	3m 41s
specs (ubuntu, 3.3.10)	1m 30s
specs (macos, 3.4.8)	1m 52s
specs (windows, 3.3.10)	3m 57s
specs (ubuntu, 4.0.1, LC_ALL=C)	1m 29s
specs (ubuntu, 3.4.8)	1m 37s
specs (ubuntu, 4.0.1)	1m 31s
specs (ubuntu, 4.0.1, --enable-frozen-string-literal)	1m 29s
	29m 29s

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