Add specs for Date#mon #1743

Job	Run time
RuboCop	25s
specs (ubuntu, 3.0.6)	1m 32s
specs (ubuntu, 3.1.4)	1m 29s
specs (ubuntu, 3.2.2)	1m 23s
specs (ubuntu, 3.3.0)	1m 29s
specs (macos, 3.0.6)	1m 28s
specs (macos, 3.1.4)	1m 53s
specs (macos, 3.2.2)	1m 53s
specs (macos, 3.3.0)	1m 12s
specs (windows, 3.0.6)	4m 25s
specs (windows, 3.1.4)	4m 20s
specs (windows, 3.2.2)	5m 44s
specs (windows, 3.3.0)	3m 56s
specs (ubuntu, 3.3.0, --enable-frozen-string-literal)	1m 23s
	32m 32s

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