* Improve default language decision
This change allows to takes account of accepted language determined by
the user agent even if the custom default locale of the instance is
configured.
* Cover Localized more
* Fix code style
* Coverage for rate limit headers
* Move rate limit headers methods to concern
* Move throttle check to condition on before_action
* Move match_data variable into method
* Move utc timestamp to separate method
* Move header setting into smaller methods
* specs cleanup
