res_pjsip_header_funcs: Make prefix argument optional.

res_pjsip_header_funcs: Make prefix argument optional.

The documentation for PJSIP_HEADERS claims that
prefix is optional, but in the code it is actually not.
However, there is no inherent reason for this, as users
may want to retrieve all header names, not just those
beginning with a certain prefix.

This makes the prefix optional for this function,
simply fetching all header names if not specified.
As a result, the documentation is now correct.

Resolves: #230

UserNote: The prefix argument to PJSIP_HEADERS is now
optional. If not specified, all header names will be
returned.

(cherry picked from commit 08e7d77fcede7b649930fba3580d938316d12a22)