Major Description
FetchSM exposes a member variable HTTPHdr client_response_hdr and anybody can modify headers in it. However, such modification breaks FetchSM's internal state, and it results unexpected behavior.
To avoid such modification, client_response_hdr shouldn't be exposed directly and FetchSM should have a getter which returns the headers with const modifier.
It sounds easy but we need to fix an other issue (TS-4350) first, because operations against const HTTPHdr are limited unreasonably.