Response format
Unless there's a lower-level error, always return 200 OK.
The Content-Type is text/xml. Content-Length must be present and correct.
The body of the response is a single XML structure, a <methodResponse>, which can contain a single <params> which contains a single <param> which contains a single <value>.
The <methodResponse> could also contain a <fault> which contains a <value> which is a <struct> containing two elements, one named faultCode, an <int> and one named faultString, a <string>.