Added spec for Enumerable#count delegating to #size

According to the ruby documentation, Enumerable#count() is supposed to delegate to #size if #size is defined ([see here](http://ruby-doc.org/core-1.8.7/Enumerable.html#method-i-count)), and fall back to #each if not. This isn't the behaviour in MRI (1.8 or 1.9) so far as I can tell.