oauth: make Google work again

Google apparently doesn't implement the spec properly?

Confirmed Camlistore's picasa importer works again after this,
and Camlistore's foursquare importer continues to work.

Going forward, we should build a manual integration test for
a dozen providers. Ideally.

LGTM=adg
R=adg, jbd
CC=golang-codereviews
https://codereview.appspot.com/129150043
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