SIP specification does not mandate the use of 'Allow'
header and which message must contain this header.
So, the 'Allow' header may be present in some SIP
messages (e.g. INVITE) but not in subsequent messages
(e.g. ACK), thus the current list of methods (if any)
will be updated only if the new list is not empty.

Gitlab: #572

Change-Id: If10c73dd116ebd2bbcec413d377caf524aca8897