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

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

```erlang
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.
```
Merge branch 'dont_overwrite_stacktrace' of git://github.com/urbanserj/cowboy