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.

- 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)