Fix any_param to work with a time attribute

When a TimeType is used, the 'ANY_PARAM' string will fail to parse (producing an 'invalid date' ArgumentError from [sunspot/type.rb:236](https://github.com/sunspot/sunspot/blob/master/sunspot/lib/sunspot/type.rb#L236)). `any_param` now responds to `.utc` with the current time.

Identified in a recent upgrade to 1.9.3.