warn on :any for methods

Thanks for rack-cors!

I wasted quite a bit of time today because I copied this example:

        resource '/public/*', :headers => :any, :methods => :get

and switched it to `:methods => :any` without a second thought.

        resource '*', :headers => :any, :methods => :any

Then spent a long time wondering why it wasn't working. This PR simply
stops people making the same mistake, and upholds the [principle of least astonishment](http://en.wikipedia.org/wiki/Principle_of_least_astonishment).