client: isolated unicode fix for Windows
client: isolated unicode fix for Windows

While tarfile documentation says that encoding defaults to 'utf-8' on Windows,
this is a lie. So force it and it seems to have magically fixed the problem.

R=9f116ddb1b24f6fc1916a676eb17161b6c07dfc1@chromium.org
Bug: 787531
Change-Id: I444fbe4dd9324440290f9baf04858788fa430dad
Reviewed-on: https://chromium-review.googlesource.com/806955
Reviewed-by: Vadim Shtayura <9f116ddb1b24f6fc1916a676eb17161b6c07dfc1@chromium.org>
Commit-Queue: Marc-Antoine Ruel <d2bf0fc09b08f7b0888b3ddab32b3e89c7122c8b@chromium.org>

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