Test: Added Tests for non-english locales

CPDateFormatter only supports the "en" locale by default. All other locales yield empty results for most symbols. The tests added ensure, that the english "default" values are returned if no specific data is available (which is currently the case for every locale except "en").
Fixed: CPDateFormatter now works for non-"en" locales

CPDateFormatter previously only defined symbols for "en"-locale. Using another locale simple yielded nil-results for most keys. This fix addresses this behavior by always returning the english symbols as default unless other symbols have been explicitly provided or set.
Merge pull request #2155 from krodelin/CPDateFormatter-emptyLanguages

Fixed: CPDateFormatter now works for non-"en" locales [+1]
Formatting: CPDateFormatter changes and tests.

Refs #2155.