Pass content_types option to Grape::Middleware::Error.

This fixes the Content-Type header on error responses with a custom
format. It wasn't able to find the custom format and used the default
of "text/html".
Merge pull request #557 from bernd/error-middleware-content-types

Pass content_types option to Grape::Middleware::Error
Workaround for Rainbow dependency, see https://github.com/bbatsov/rubocop/pull/763.
Fix deprecation warning: I18n.enforce_available_locales will default to true in the future.
Support lambda-based values for params.
Added date of the 0.6.1 release.
add ability to use Grape::Entity documentation in the params block
add test for using Grape::Entity documentation in requires