subprocess42: be more tolerant to unicode.
subprocess42: be more tolerant to unicode.

Make it automatically encode unicode strings to str as UTF-8. This simplifies
everyone's life. Asserting that all call sites are using str would be painful
and labour intensive so it's not worth.

R=9f116ddb1b24f6fc1916a676eb17161b6c07dfc1@chromium.org
Bug: 787531
Change-Id: Ib683dbccad98d4d57f1ebd5bd627d4b1874062b2
Reviewed-on: https://chromium-review.googlesource.com/798150
Commit-Queue: Marc-Antoine Ruel <d2bf0fc09b08f7b0888b3ddab32b3e89c7122c8b@chromium.org>
Reviewed-by: Vadim Shtayura <9f116ddb1b24f6fc1916a676eb17161b6c07dfc1@chromium.org>

luci/luci-py,luci/luci-py,luci/luci-py,luci/luci-py