Code coverage for custom headers, including checking that cloning doesn't leak memory
Prevent quadruple-free error on shutdown
Merge pull request #688 from Metaswitch/crash_on_shutdown

[Reviewer: Andy] Prevent quadruple-free error on shutdown
Merge pull request #687 from Metaswitch/custom_header_coverage

[Reviewer: Andy] Code coverage for custom headers, including checking that cloning doesn'...