Add some missing invite state changes necessary in the sip_write

Add some missing invite state changes necessary in the sip_write
function. Not setting the invite state correctly on the call was
resulting in the Record application leaving empty files. I also
have updated the doxygen comment next to the declaration of the
INV_EARLY_MEDIA constant to reflect that we also use this state
when we *send* a 18X response to an INVITE.

(closes issue #13878)
Reported by: nahuelgreco
Patches:
      sip-early-media-recording-1.4.22.patch uploaded by nahuelgreco (license 162)
	  Tested by: putnopvut

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/1.4@157503 65c4cc65-6c06-0410-ace0-fbb531ad65f3