Fix for parsing of headers with empty values

* Also added a toString to SipPacket.
* Fix for parsing header with empty values

The following wouldnt parse correctly:

X-Whatever: \r\n
Call-ID: asdfasdf

The value of X-Whatever became "Call-ID: asdfasdf" which obviously isnt correct. Im not 100% convinced this is even legal but doesnt matter since this is happening in the wild so have to deal with it.