Fix BadRequestException when using 'service_account_token' field.
Fix BadRequestException when using 'service_account_token' field.

This error was introduced in the previous CL.

TBR=maruel@chromium.org
BUG=731847

Change-Id: Ic26028a120eac0a560da54543efd41b5ffed17d0
Reviewed-on: https://chromium-review.googlesource.com/614783
Reviewed-by: Vadim Shtayura <9f116ddb1b24f6fc1916a676eb17161b6c07dfc1@chromium.org>
Commit-Queue: Vadim Shtayura <9f116ddb1b24f6fc1916a676eb17161b6c07dfc1@chromium.org>

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