libavformat/http: add support for content_type option in listen mode

Instead of silently ignoring the content_type option in listen mode,
apply its value to the provided "Content-Type:" header.
Signed-off-by: Moritz Barsnick <barsnick@gmx.net>
Reviewed-by: Nicolas George <george@nsup.org>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

doc/protocols.texi

@@ -276,7 +276,7 @@ value is -1.
 If set to 1 use chunked Transfer-Encoding for posts, default is 1.
 
 @item content_type
-Set a specific content type for the POST messages.
+Set a specific content type for the POST messages or for listen mode.
 
 @item http_proxy
 set HTTP proxy to tunnel through e.g. http://example.com:1234

libavformat/http.c

@@ -355,7 +355,7 @@ static int http_write_reply(URLContext* h, int status_code)
     case 200:
         reply_code = 200;
         reply_text = "OK";
-        content_type = "application/octet-stream";
+        content_type = s->content_type ? s->content_type : "application/octet-stream";
         break;
     case AVERROR_HTTP_SERVER_ERROR:
     case 500: