An XDI client that can send XDI messages over HTTP and receive results.
It supports the following parameters (passed to the init method):
endpointUrl - The URL of the XDI endpoint to talk to.
sendMimeType - The mime type to use to send the XDI messages to the endpoint. The Content-type header will be set accordingly.
recvMimeType - The mime type in which we want to receive the results from the endpoint. The Accept header will be set accordingly.
If the endpoint replies in some other mime type than requested, we will still try to read it.