* reverseproxy: Keep own headers after 1xx responses
Since #6656 the 1xx handler clears the whole header map after writing the
informational response, which also drops headers set before proxying (for
example by the header handler). Restore those after clearing, as #5091 did.
* reverseproxy: Use maps.Copy to restore own headers
* reverseproxy: Check the 103 reaches the client in the 1xx headers test