The SDP answer is included in non-realiable 1XX response, if
NUTAG_EARLY_ANSWER(1) is included in nua_response() tags, or the user SDP is
specified with SOATAG_USER_SDP() or SOATAG_USER_SDP_STR() in nua_response()
tags.

test_basic_call.c: added test_basic_call_2() for testing the call setup
where 180 contains the answer and the SDP in 200 OK is ignored.

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