key as translation for defaultLang if file exists

The current behavior is to add new translations as empty string even for the default language. With this patch it will allways put the key as translation for the defaultLang.
Merge branch 'patch-1' of git://github.com/wilgert/grunt-angular-translate into wilgert-patch-1
Update tests to handle defaultLang behavior
Merge branch 'wilgert-patch-1'

Fix testing