Signed-off-by: Tibor Vass <tibor@docker.com>
(cherry picked from commit bd8f49e)

Signed-off-by: Tibor Vass <tibor@docker.com>
(cherry picked from commit aa3ad5f)

Signed-off-by: Tonis Tiigi <tonistiigi@gmail.com>
(cherry picked from commit bc3a1ee)

Signed-off-by: Tonis Tiigi <tonistiigi@gmail.com>
(cherry picked from commit 1d7e7a6)

If a single ETag is requested in `If-None-Match`, some servers do not
include that (unambiguous) ETag header in the response.

For detailed description, see:
moby#905 (comment)

Signed-off-by: Robert Estelle <robertestelle@gmail.com>
(cherry picked from commit c63c6f9)

Otherwise a 200 response without an ETag could be incorrectly associated
to previous content in the following scenario:

* The remote server had in the past responded with an ETag for this
  resource, which was cached.
  - (Otherwise, onlyETag would be empty)
* That was the only ETag cached for this resource.
  - (Otherwise, onlyETag would be empty)
* The remote server then stopped supporting ETag/If-None-Match for this
  resource at all.
  - (Otherwise, it would respond with a 304 or a 200+ETag)

Signed-off-by: Robert Estelle <robertestelle@gmail.com>
(cherry picked from commit 6ff0868)

Signed-off-by: Tonis Tiigi <tonistiigi@gmail.com>
(cherry picked from commit eede0fa)

Signed-off-by: Tonis Tiigi <tonistiigi@gmail.com>
(cherry picked from commit e3e13ad)