The allowed_digests option were added to the formpost middleware in
addition to the tempurl middleware[1], but the option was not added to
the formpost section in the example proxy config file.
[1] 2d063cd61f6915579840a41ac0248a26085e0245
Change-Id: Ic885e8bde7c1bbb3d93d032080b591db1de80970