Move body length count to cowboy_stream_h instead of protocols

The documentation was correct, the code was not.

This should make it easier to implement new protocols. Note that
for HTTP/2 we will need to add some form of counting later on to
check for malformed requests, but we can do simpler and just
reduce from the expected length and then check if that's 0 when
IsFin=fin.

src/cowboy_req.erl

@@ -440,7 +440,7 @@ read_body(Req=#{pid := Pid, streamid := StreamID}, Opts) ->
 	receive
 		{request_body, Ref, nofin, Body} ->
 			{more, Body, Req};
-		{request_body, Ref, {fin, BodyLength}, Body} ->
+		{request_body, Ref, fin, BodyLength, Body} ->
 			{ok, Body, set_body_length(Req, BodyLength)}
 	after Timeout ->
 		exit(timeout)