Glance cache ignores range request headers

When attempting to download a part[1] of an image, the glance cache always returns the entire image. Neither the 'Range' nor the 'Content-Range' headers are not accounted for.

Both requests below aims to get the first 50 bytes of an image, but instead the complete image is returned with a 200 OK status.
curl ${glance_host}/v2/images/:image_id/file -H 'Range: bytes=0-49'
curl ${glance_host}/v2/images/:image_id/file -H 'Content-Range: bytes 0-49/*'

This will become more of an issue once #1399851 is solved.

Produced on a devstack install, at commit bcc772 of glance.

[1]: https://tools.ietf.org/html/rfc7233#section-4