Cannot change default behavior when using respond_with

Hello,

First, thank you for your awsome gem!
I'm using really 

I found a problem when user users respond_with instead of respond_to.
http://api.rubyonrails.org/classes/ActionController/Responder.html

User uses responder style, all response block and options will be ignored.

For example in controller
``` ruby
def index
    
end 
```





Patch description follows;

Fix responder method to keep comaptibility with default rails
responder style. These are needed if user uses responders.

 * Accept render options
 * Call user block when block is specified