Throw notices when invalid date values are passed to WP_Date_Query.

`_doing_it_wrong()` notices are now generated when passing out-of-range values
(`month=13`) or invalid dates (`2014-02-29`).

Includes unit tests.

Props ChriCo.
Fixes #25834.

git-svn-id: https://develop.svn.wordpress.org/trunk@29925 602fd350-edb4-49c9-b593-d223f7449a82