Make sure we don't call should_be_throttled() too much (bug 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)
Merge pull request #1339 from diox/throttling-fun

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