qt-brisa: fix not inited value
qt-brisa: fix longstanding crash, double free

There were several attempts to fix this bug:

10d446c8 - qt-brisa: fix crash on free
 and
4f7210f2 - qt-brisa: change qobjects cleanups policy

all of these patches are extremely useful, but the original
bug was in double free because of not implemented assignment
operator.

1. assignment operator for httpmessage must be implemented, which
   takes ownership of the m_entityBody object

2. firstly do request and only then copy it.
qt-brisa: set explicit content type to 'text/xml' for brisa service description xml
qt-brisa-mediaserver: fix wrong param, should videoRoot not audio