Stop defining an empty Date class

* Simply use a matcher: if we have an instance of Date,
  Date must be loaded and we can simply use Date#===.
  If Date is not defined, then Date is not loaded
  and we can not have an instance of Date.
* See https://bugs.ruby-lang.org/issues/9890
Merge pull request #948 from eregon/no_empty_date

Stop defining an empty Date class