Make case insensitive comparisons when translating month names

Makes this

````
irb(main):002:0> I18n.with_locale(:es) { Date.parse_localized('8 agosto 2015') }
Sat, 08 Aug 2015
````
and this

````
irb(main):001:0> I18n.with_locale(:es) { Date.parse_localized('8 Agosto 2015') }
Sat, 08 Aug 2015
````

work