Allow concerns defined as a string array

Right now several of the route options (only, except) can be defined
as arrays of string or symbols. I was surprised when playing with
rails 4 and this didn't work for concerns.

- Added test for string array declaration.

With this change concerns could be declared with strings or symbols:

    resources :posts, concerns: [:commentable, :taggable]
    resources :media, concerns: %w(commentable taggable)

__This includes changes mentioned in an original pull request: https://github.com/rails/rails/pull/9725__