This is a reland of 5bfa3ae8

Replace cStringIO with StringIO and add tests.

Original change's description:
> gerrit_util: Refactor ReadHttpResponse and add more tests.
>
> Bug: 1016601
> Change-Id: Ie6afc5b1ea29888b0bf40bdb39b2b492d2d0494c
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1880014
> Reviewed-by: Anthony Polito <apolito@google.com>
> Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>

Bug: 1016601
Change-Id: I0c83a83202169b6a1acc60bdf6f4cd00eac6e2a6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1884461Reviewed-by: Anthony Polito <apolito@google.com>
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>