duplicating Content-Type header

[dotysan]

This wasn't a problem in Python 2 where xmlrpclib used connection.putheader("Content-Type", "text/xml"). But in Python 3, where xmlrpc.client uses headers.append(("Content-Type", "text/xml")), this https://github.com/joshmarshall/jsonrpclib/blob/master/jsonrpclib/jsonrpc.py#L107 connection.putheader("Content-Type", "application/json-rpc") actually creates a second Content-Type header instead of replacing the text/xml one.

This duplicate Content-Type headers generates 400s on lighttpd.

[dotysan]

*Second Content-Type header; not duplicate.