[Reviewer: Mike] Invalidate cached message info whenever we manually change the response code

Mike,

Please can you review this simple fix to invalidate cached message information whenever we manually change the response code of the message.  This fixes #456, where we log out-of-date information.

Note that pjsip_tx_data_invalidate_msg invalidates both the cached message summary (which we need to be invalidated) and the actual constructed message (which we don't need to be invalidated, but doesn't hurt because the constructed message is either empty already or will be invalidated by PJSIP before it is sent).

I've run UTs and live tests.

Matt