Always merge I18n format values, namespaced or not, over the default
ones, to ensure I18n format defaults will have precedence over our
namespaced values.
Precedence should happen like this:
default :format
default :namespace :format
i18n :format
i18n :namespace :format
Because we cannot allow our namespaced default to override a I18n
:format config - ie precision in I18n :format should always have higher
precedence than our default precision for a particular :namespace.
Also simplify default format options logic.
Action Pack already comes with a default locale fine for :en, that is
always loaded. We can just fallback to this locale for defaults, if
values for the current locale cannot be found.
Closes#4420, #2802, #2890.
