REFER should have sips contact if far end is using sips (#168)

lib/call-session.js

@@ -1173,7 +1173,7 @@ Duration=${payload.duration} `
       // eslint-disable-next-line no-unused-vars
         'content-length': _contentlength, 'refer-to': _referto, 'referred-by': referredby,
         // eslint-disable-next-line no-unused-vars
-        'X-Refer-To-Leave-Untouched': _leave,
+        'X-Refer-To-Leave-Untouched': _leave, contact: _contact,
         ...customHeaders
       } = req.headers;
 
@@ -1193,6 +1193,7 @@ Duration=${payload.duration} `
           headers: {
             'Refer-To': `<${stringifyUri(uri)}>`,
             'Referred-By': referredby,
+            'Contact': this.contactHeader,
             ...customHeaders
           }
         });