Add specs for String#encode :fallback option #1646

Job	Run time
RuboCop	24s
specs (ubuntu, 3.0.6)	1m 28s
specs (ubuntu, 3.1.4)	1m 30s
specs (ubuntu, 3.2.2)	1m 26s
specs (ubuntu, 3.3.0)	1m 24s
specs (macos, 3.0.6)	1m 23s
specs (macos, 3.1.4)	1m 54s
specs (macos, 3.2.2)	1m 12s
specs (macos, 3.3.0)	1m 22s
specs (windows, 3.0.6)	3m 29s
specs (windows, 3.1.4)	4m 47s
specs (windows, 3.2.2)	4m 36s
specs (windows, 3.3.0)	4m 52s
specs (ubuntu, 3.3.0, --enable-frozen-string-literal)	1m 31s
	31m 18s

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