Workaround a bug in task queue's stub.
Workaround a bug in task queue's stub.

For some reason, it started to return unicode values for task request
headers. It breaks internal assertion in webtest code.

R=d2bf0fc09b08f7b0888b3ddab32b3e89c7122c8b@chromium.org

Change-Id: Iba5399e45b98f86c04983f655716a81577e56507
Reviewed-on: https://chromium-review.googlesource.com/1250036
Commit-Queue: Marc-Antoine Ruel <d2bf0fc09b08f7b0888b3ddab32b3e89c7122c8b@chromium.org>
Reviewed-by: Marc-Antoine Ruel <d2bf0fc09b08f7b0888b3ddab32b3e89c7122c8b@chromium.org>

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