Fix customParseFormat monthIndex calculation for december and "format" month strings #2942
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Alternative to #2251 to fix #2249. Adjust the bounds check instead of introducing a helper function. Added test cases making use of existing locale data.
Had to use belarussian locale because that's the only one with a
monthsShortwhere the string for december actually differs between format and standalone variants.Fixes #2249
Fixes #2483
Fixes #2511