Respond w/ xml on errors when accept header set to application/xml

I thought it would be nice if the proxy would respond with xml (for error responses) when the client has set the request accept header to application/xml

We are using apiaxle as a gateway for our api which accepts and provides payloads in either json or xml format based on the requester's accept header.  With this modification, our clients will also receive any key/throttling related errors in their desired accept content-type.