This creates a (successful) XML-RPC methodResponse and returns it as a string.
To create a fault response, use build_fault. This is the low-level method that message_set_response is built on.
See build_request for serialization details, but note that since a method can
only have a single return value, value should not be a tuple here (unless the return value is an array).
If value is floating, it is consumed.
| value |
the return value |
|
the text of the methodResponse, or |