The authentication errors were not sent as the connection was closed
immediately. The reason for this was the fact that if a client request
uploaded data with bad credentials, MaxScale would not send a response if
the connection was kept open. Closing the socket solved the hang but
caused confusing errors on the client side.
The libmicrohttpd library appears to require full processing of any data
uploaded by a client request before a request can be sent. With this
change, the clients receive proper authentication errors in all cases.
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