Don't overwrite stackstace in `cowboy_rest:error_terminate/2`

This stacktrace is very useful in the `onresponse` hook. For example:

	internal_error_hook(500, Headers, <<>>, Req) ->
	    StackTrace = erlang:get_stacktrace(),
	    Headers0 = [{N, V} || {N, V} <- Headers, N =/= <<"content-length">>],
	    Body = io_lib:format("~p", [StackTrace]),
	    {ok, Req0} = cowboy_req:reply(500, Headers0, Body, Req),
	    Req0;
	internal_error_hook(Status, Headers, Body, Req) ->
	    {ok, Req0} = cowboy_req:reply(Status, Headers, Body, Req),
	    Req0.