Add specs for Data#to_h #276

Job	Run time
RuboCop	27s
specs (ubuntu, 3.0.6)	1m 27s
specs (ubuntu, 3.1.4)	1m 28s
specs (ubuntu, 3.2.2)	1m 25s
specs (ubuntu, 3.3.0)	1m 22s
specs (macos, 3.0.6)	1m 24s
specs (macos, 3.1.4)	1m 19s
specs (macos, 3.2.2)	1m 12s
specs (macos, 3.3.0)	1m 12s
specs (windows, 3.0.6)	4m 14s
specs (windows, 3.1.4)	5m 28s
specs (windows, 3.2.2)	5m 30s
specs (windows, 3.3.0)	3m 36s
specs (ubuntu, 3.3.0, --enable-frozen-string-literal)	1m 26s
	31m 30s

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