Also call stream.update on the remaining streams when removing streams

Ignore-this: ee22dbdc6169a47182785913a22c1066

darcs-hash:20141125150048-56b4d-ddf6c7464bbbd1109b445f0a2c57b4bb16f09530
Extend Session.connect and Session.accept with an extra_headers parameter

Ignore-this: 8734e49284e2d2ec99bd4fc2267df822

As a result, Session.connect and the session itself loses the 'subject' attribute,
and SIPSessionNewIncoming notification will get a 'headers' parameter which the user
can use to inspect what the session subject is.

darcs-hash:20141125151735-56b4d-facf724a969fbdd01237d0677a07ef2da7c88903
Improved test statements

Ignore-this: 66af7407e11d17441da4a9853598b713

darcs-hash:20141126022616-b22ec-24b2cfb1161d35860ee45646272ae5fa93eb4b61
Fixed header name

Ignore-this: 72afc8173ac3e3a84382e229b7e6b03c

darcs-hash:20141126022659-b22ec-34657f46d712a842ebb883841d06a54ab90c26f2