Bug 6335 Set branch in sip session

Bug 6335 Set branch in sip session

The server returns its branch as the institution in the config file
However in transactions it will default to the homebranch of the
sip user which may not be consistent with that. Needs to set branch
in the session if there is a value in config otherwise statistics and
behaviour can be wrong (esp in discharge)

Signed-off-by: Chris Cormack <chrisc@catalyst.net.nz>
Signed-off-by: Paul Poulain <paul.poulain@biblibre.com>
Signed-off-by: Chris Cormack <chrisc@catalyst.net.nz>