Fixed segmentation fault error during destroying of QHttpConnection
Prefer using Q_SLOTS and Q_SIGNALS macros

... to prevent from side-effects with other libs as suggest in Qt documentation
Act as a "add" instead of a "set"
Send the 'done' signal on connection close

This allows the application to know when the client unexpectedly closed the connection and then avoid to access the QHttpResponse after it has been deleted by the server object.
Added "flush" and "waitForBytesWritten" methods to QHttpResponse