olbrich/ruby-units — pull request #89 by JoeStanton (2013-11-27 00:30:59+00:00)
Title: Test for zero? before using to fix Unit <=> string comparisons

Previously, comparisons such as the following would fail:

```ruby
Unit('500MB') > "100MB"
```

This is because of a #zero? method call on the previous case, which
isn't implemented on string for obvious reasons.

Checking for it's existence with #respond_to? seemed an easy fix.