Fixes CallOptions SIP authentication

I have updated CallOptions - SipAuthUsername and SipAuthPassword are strings and they are included in the AddCallOptions method.

According to Twilio docs:
Example 2 shows that properties should be of type string and the intended usage:
http://www.twilio.com/docs/api/rest/sip#example-2
Optional parameters section describes the parameters.
http://www.twilio.com/docs/api/rest/sip#post-parameters-optional