Always compute Enumerable#count through enumeration for 1.9+

Looks like MRI [dropped the `#size` optimization in `#count`](https://github.com/ruby/ruby/commit/c0a0aa0c47f2cd5c97a35effb8b073eeb84b7d2d) in 1.9.2.  Comparing the [1.8 doc](http://ruby-doc.org/core-1.8.7/Enumerable.html#method-i-count) to the [2.0 doc](http://ruby-doc.org/core-2.0.0/Enumerable.html#method-i-count) also shows this is the intended behavior (interestingly, the [1.9 doc](http://ruby-doc.org/core-1.9.3/Enumerable.html#method-i-count) seems to be out of date)

Update to match that behavior (also fixes #922).