Caching policy does not work when combined with the batching one

The caching policy does not work correctly when combined with the 3scale batcher one.

The reason is that there was a bug in "Policy.export()", the function that policies use to expose data to other policies. Because of the bug, the data was exported correctly only for the rewrite phase.

One of the consequences of this bug was that the caching policy was not working well when combined with the 3scale batcher one.

Reference: https://github.com/3scale/APIcast/pull/1023