SIP MESSAGE method is an extention to SIP methods and might not be
supported by all User Agents (UA).
In this change, the MESSAGE method is only used if it's present in
the list of allowed methods (Allow header).
Since the Allow header is not mandatory, if the Allow header is not
present we assume that the MESSAGE method is not supported.

Gitlab: #572

Change-Id: I85d35fa0f9be108b34a538b8c732f1530ad68bd6