Update invite options when receiving INVITE provisional response with Allow UPDATE header

[riza]

As stated in ​rfc3261

13.2.2.1 1xx Responses

   Zero, one or multiple provisional responses may arrive before one or
   more final responses are received.  Provisional responses for an
   INVITE request can create "early dialogs".  If a provisional response
   has a tag in the To field, and if the dialog ID of the response does
   not match an existing dialog, one is constructed using the procedures
   defined in Section 12.1.2.

   The early dialog will only be needed if the UAC needs to send a
   request to its peer within the dialog before the initial INVITE
   transaction completes.  Header fields present in a provisional
   response are applicable as long as the dialog is in the early state
   (for example, an Allow header field in a provisional response
   contains the methods that can be used in the dialog while this is in
   the early state).

And ​rfc3311:

   When a UAS compliant to this specification receives an INVITE request
   for a new dialog, and generates a reliable provisional response
   containing SDP, that response SHOULD contain an Allow header field
   that lists the UPDATE method.  This informs the caller that the
   callee is capable of receiving an UPDATE request at any time.  An
   unreliable provisional response MAY contain an Allow header field
   listing the UPDATE method, and a 2xx response SHOULD contain an Allow
   header field listing the UPDATE method.

This patch will update invite options when Allow UPDATE header is included in the provisional response of the INVITE.

Thanks to Pirmin (pirmin@walthert.net) for the patch

[riza]

In 5921:

Close #2169: Update invite options when receiving INVITE provisional response with Allow UPDATE header.