Fix: requests with body missing content-type header

Author: mtyx

src/cowboy_reverse_proxy.erl

@@ -130,7 +130,7 @@ request(Req, Opts) ->
   RequestHeaders = request_headers(Req, Opts),
   case cowboy_req:has_body(Req) of
     true ->
-      ContentType = to_string(cowboy_req:header("content-type", Req, "")),
+      ContentType = to_string(cowboy_req:header(<<"content-type">>, Req, "")),
       {BodyReq, Body} = request_body(Req, Opts),
       {BodyReq, {RequestURL, RequestHeaders, ContentType, Body}};
     false ->