Fixes image-download error of v2

When trying to retrieve a Glance image using the v2 api, user will
get an error:

'utf8' codec can't decode byte 0xcc in position 1217: invalid continuation byte

This is because Glance client expects a response of type content-type:
application/octet-stream but it's getting a Content-Type:

text/html; charset=UTF-8 Transfer-Encoding: chunked

The root cause is Glance will return the image data from cache if
there is. However, in method _process_v2_request, the content type
has never been configured. See:
https://github.com/openstack/glance/blob/master/glance/api/middleware/cache.py#L162

Fixes bug 1193310

Change-Id: Icab5e28db63455358ab3919bd94c484411987e39
Merge "Fixes image-download error of v2"