Fix caching under pip 9

[Roguelazer]

As far as I can tell, pip 9 requires all of the following to cache a downloaded file in the local HTTP cache:

• link includes #md5=
• ETag set to a value matching the md5
• Date and Cache-Control headers set appropriately

I haven't tested this under 1.3.0 yet, but we're running this locally applied against 1.2.2 and it fixed HTTP response caching for us.

[Roguelazer]

(sorry, the original branch included a spurious commit; I've rebased it out)

[mardiros]

Thanks for the work!