Add spec around calling Symbol#to_proc for a method that yields

A proc generated by a `Symbol#to_proc` can accept and yield to a block passed by calling `call` on the proc in Ruby 1.9 (tested in 1.9.2 & 1.9.3).

This test fails on JRuby 1.7.2 & Rubinius nightly (both in 1.9 mode). [Found](https://travis-ci.org/amarshall/block_chain/builds/4761681) during the for-fun development of [BlockChain](https://github.com/amarshall/block_chain).