retrocompatibility issue with automatic contact generation in invite

[flaviogrossi]

We are having issues using this library with software previously compiled using the original sofia-sip hosted at sourceforge.

In particulare there is an issue with automatic contact generation during an invite. It seems that commit afb119a is not retrocompatible and broke some existing software. Also, it seems to go against the documentation which states that a contact header is generated if the library user does not provide one esplicitly and does not pass the SIPTAG_CONTACT(NULL) option.

One example is the janus webtrc gateway which seems to not be compatible with this repo, while working with the original sofia-sip library. This is the relevant issue. The problem there is that the contact header is now missing, since it relied on the sofia stack to automatically generate it (as per the docs). Everything is fixed by reverting the aformentioned commit.

Any thoughts? What is the rationale of the change?

Thanks

EDIT: this is related to #25