Specify that DefineExplicitConversions defines nil conversion methods

This change to the specs more accurately reflects the implementation,
which now delegates all of these methods to nil. This change also allows
the specs to run (and pass) on Ruby 1.9 without any conditional logic.
Add to_enum to the list of defined explicit conversions
Run specs in random order to catch order-dependency bugs
Make trailing whitespace consistent across all files
Use the new expect(...).to receive syntax
Delegate explicit conversion methods via method_missing