HashParser#schedule handles a nil value for @hash

In a Rails application I'm serializing an attribute, `schedule`, using the `Icecube::Schedule` class. I'm unable to load any objects where the `schedule` has not been set, i.e. it has a `nil` value. This is due to a `TypeError` that occurs within `HashParser#to_schedule` method. 

When Rails attempts to load an ActiveRecord object that has a serialized `IceCube::Schedule` class it calls `IceCube::Schedule.load`, https://github.com/wallace/ice_cube/blob/master/lib/ice_cube/schedule.rb#L386.  The call chain eventually calls `HashParser#to_schedule`, https://github.com/wallace/ice_cube/blob/master/lib/ice_cube/schedule.rb#L343 with a `nil` value for `@hash` which results in a `TypeError` "can't dup NilClass" when `HashParser` attempts to normalize the keys.

This pull request ensures that `to_schedule` handles a `nil` value in the `@hash` by default to an empty hash when `@hash` is `nil`. I'd prefer to use `to_h` but that's not available in Ruby 1.9.3. :(