avformat/http: increase BUFFER_SIZE to MAX_URL_SIZE + HTTP_HEADERS_SIZE

avformat/http: increase BUFFER_SIZE to MAX_URL_SIZE + HTTP_HEADERS_SIZE

Some real-world sites use an authorization header with a bearer token; when
combined with lengthy request parameters to identify the video segment,
it's rather trivial these days to have a request body of more than 4k bytes.

MAX_URL_SIZE is hard-coded to 4k bytes in libavformat/internal.h, and
HTTP_HEADERS_SIZE is 4k as well in libavformat/http.h, so this patch increases
the buffer size to 8k, as that is the default request body limit in Apache, and
most other httpds seem to support at least as much, if not more.

Signed-off-by: Marton Balint <cus@passwd.hu>