http: Consider the stream as seekable if the reply contains Accept-Ranges: bytes

The initial request contains "Range: 0-", which servers normally
have responded with "HTTP/1.1 206 Partial Content" reply with
a Content-Range header, which was used as indicator for seekability.

Apache, since 2.2.20, responds with "HTTP/1.1 200 OK" for these
requests, which is more friendly to caches and proxies, but the
seekability still is indicated via the Accept-Ranges: bytes header.
Signed-off-by: Martin Storsjö <martin@martin.st>

libavformat/http.c

@@ -265,6 +265,8 @@ static int process_line(URLContext *h, char *line, int line_count,
                     s->filesize = atoll(slash+1);
             }
             h->is_streamed = 0; /* we _can_ in fact seek */
+        } else if (!strcasecmp(tag, "Accept-Ranges") && !strncmp(p, "bytes", 5)) {
+            h->is_streamed = 0;
         } else if (!strcasecmp (tag, "Transfer-Encoding") && !strncasecmp(p, "chunked", 7)) {
             s->filesize = -1;
             s->chunksize = 0;