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 and content-type headers.  With this modification, our clients will also receive any key/throttling related errors in their desired accept content-type.