metadata[:blocks] array is growing on every request

Serious bug, when metadata[:blocks] array is growing on every request in a huge amount of times, that cause a memory leaks and callback duplicate problems. Steps to reproduce:

* Call add_metadata{p :hello} at any resource once,
* Render a resource several times,
* :hello will output to a console for a growing number of times!

I'm not sure, how to provide tests for that, sorry...