update type of resp_body to iodata

resp_body is set by cowboy_http_req:set_resp_body/2 which accepts
the input type iodata(). Update the resp_body field to use the same
type. Also update the cowboy_http_req:has_resp_body/2 function to
use the iolist_size/1 which works with both binary() and iolist()
values instead of the byte_size/1 function which only works with
binary() values.

include/http.hrl

@@ -68,7 +68,7 @@
 	%% Response.
 	resp_state = waiting   :: locked | waiting | chunks | done,
 	resp_headers = []      :: http_headers(),
-	resp_body  = <<>>      :: binary(),
+	resp_body  = <<>>      :: iodata(),
 
 	%% Functions.
 	urldecode :: {fun((binary(), T) -> binary()), T}

src/cowboy_http_req.erl

@@ -416,7 +416,7 @@ has_resp_header(Name, #http_req{resp_headers=RespHeaders}) ->
 %% @doc Return whether a body has been set for the response.
 -spec has_resp_body(#http_req{}) -> boolean().
 has_resp_body(#http_req{resp_body=RespBody}) ->
-	byte_size(RespBody) > 0.
+	iolist_size(RespBody) > 0.
 
 %% @equiv reply(Status, [], [], Req)
 -spec reply(http_status(), #http_req{}) -> {ok, #http_req{}}.