Respond with allowed methods.

Minor improvement: When a group of routes for a certain path does not support a certain method, include the allowed request methods in the response via the 'Allow' header.
As per RFC2616 on 405 responses "The response MUST include an Allow header
containing a list of valid methods for the requested resource."

I was not sure whether to augment the `respond` function with a parameter for headers or whether to just use modifyResponse (I chose the latter).