Add a function to lock request responses instead of inlining

Just more cleanup in order to have req() opaque.

src/cowboy_req.erl

@@ -103,6 +103,7 @@
 
 %% Misc API.
 -export([compact/1]).
+-export([lock/1]).
 -export([to_list/1]).
 -export([transport/1]).
 
@@ -908,6 +909,12 @@ compact(Req) ->
 		bindings=undefined, headers=[],
 		p_headers=[], cookies=[]}.
 
+%% @doc Prevent any further responses.
+%% @private
+-spec lock(Req) -> Req when Req::req().
+lock(Req) ->
+	Req#http_req{resp_state=locked}.
+
 %% @doc Convert the Req object to a list of key/values.
 -spec to_list(req()) -> [{atom(), any()}].
 to_list(Req) ->