Add spec for String#to_f with non-ascii compatible encoding #387

Job	Run time
specs (ubuntu, 3.1.6)	1m 33s
RuboCop	32s
specs (ubuntu, 3.2.7)	1m 29s
specs (ubuntu, 3.3.7)	1m 32s
specs (ubuntu, 3.4.2)	1m 35s
specs (macos, 3.1.6)	1m 25s
specs (macos, 3.2.7)	1m 32s
specs (macos, 3.3.7)	1m 31s
specs (macos, 3.4.2)	1m 41s
specs (windows, 3.1.6)	3m 16s
specs (windows, 3.2.7)	4m 37s
specs (windows, 3.3.7)	4m 6s
specs (windows, 3.4.2)	5m 8s
specs (ubuntu, 3.3.0, --enable-frozen-string-literal)	58s
	30m 55s

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