Return a 0 vs empty string in Mixed Number formatter when a 0 is passed. Fixes #1
Using a guard function vs a wraping if statement.
Convert specs to RSpec 3.0.2 syntax with Transpec

This conversion is done by Transpec 2.3.1 with the following command:
    transpec

* 45 conversions
    from: obj.should
      to: expect(obj).to

For more details: https://github.com/yujinakayama/transpec#supported-conversions