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>