Make sure we don't call should_be_throttled() too much (bug 932908)

https://bugzilla.mozilla.org/show_bug.cgi?id=932908

We don't need to call throttling for GET requests (it would prevent all usage of the Marketplace) and we don't need to call the method twice if the identifiers are the same (in our case, it's currently always the IP for all authentication classes)