res_pjsip_diversion: fix double 181

Arming response to both AST_SIP_SESSION_BEFORE_REDIRECTING and
AST_SIP_SESSION_BEFORE_MEDIA causes 302 to to be handled twice,
resulting in to 181 being generated.

Change-Id: I866e5461564644ffb8a5e12b6f1330b50a7b63ab