Fix #85. Do not add ';ob' in Contact of inital INVITE if we got a GRUU URI

RFC 5626 Section 4.3 "Sending Non-REGISTER Requests":

"If the UAC is using a Globally
Routable UA URI (GRUU) [RFC5627] that was instantiated using a
Contact header field value that included an "ob" parameter, the UAC
sends the request over the flow used for registration, and subsequent
requests will arrive over that same flow.  If the UAC is not using
such a GRUU, then the UAC adds an "ob" parameter to its Contact
header field value."