res_pjsip: set Accept-Encoding to identity in OPTIONS response

RFC 3261 says that the Accept-Encoding header should be present
in an options response. Permitted values according to RFC 2616
are only compression algorithms like gzip or the default identity
encoding. Therefore "text/plain" is not a correct value here.
As long as the header is hard coded, it should be set to "identity".

Without this fix an Alcatel OmniPCX periodically logs warnings like
"[sip_acceptIncorrectHeader] Header Accept-Encoding is malformed"
on a SIP Trunk.

ASTERISK-29165 #close

Change-Id: I0aa2211ebf0b4c2ed554ac7cda794523803a3840