Add test for new function
Copy P-Charging-Vector/P-Charging-Function-Addresses through to REGISTER 200 OKs and third-party REGISTERs
Test that SUBSCRIBE responses have charging headers
Code review feedback - use existing constants, copy charging headers on responses more generally
Merge pull request #445 from Metaswitch/charging_headers_on_register

[Reviewer: Andy] Charging headers on REGISTER responses/third-party REGISTERs