couchdb-glib should export couchdb_session_send_message

[Ethan Glasser-Camp]

Although couchdb_session_send_message is documented and a warning says it "should not be used by applications unless they really have a need (like missing API in couchdb-glib which the application needs)", the function is not declared in couchdb-session.h, so client programs cannot use it (unless they declare the function themselves). This function is useful so please add it.