Including one more spec for verifying proxy to make sure allow/receive expectations work

This is a complement to #623, this validates that:

    allow(object).to receive(:implemented) { :value }

Correctly evaluates the block given as the implementation.
Merge pull request #628 from mauricio/spec-for-allow-receive-on-verifying-proxy

Including one more spec for verifying proxy to make sure allow/receive expectations work