handle head method when get is defined

Fix for https://github.com/koajs/route/issues/17.

Here is what the documentation says:

> The HEAD method is identical to GET except that the server MUST NOT return a message-body in the response. The metainformation contained in the HTTP headers in response to a HEAD request SHOULD be identical to the information sent in response to a GET request. This method can be used for obtaining metainformation about the entity implied by the request without transferring the entity-body itself.

Based on the description above the module works as expected and this is not a bug. Since `koa` core handles HEAD, as @visionmedia mentioned, I created the PR anyway (useful).