sip: fix attended transfers

Attended transfers led to an assertion in pjsip. According to high-level
librairie pjsua, unused in Ring, and specially function pjsua_call_on_incoming,
it looks like we should not call pjsip_inv_answer when replace header is
present. This function actually leads to the pjsip assertion.
pjsip_inv_initial_answer should be rather used.

NOTIFY is still missing.

Change-Id: I2e4a09533bd33dd32b2cc61a7d9737dc1fe79738
Tuleap: #122