Better "inclusive" support for string values in WP_Date_Query.

The 'inclusive' parameter for WP_Date_Query determines whether non-precise
dates for 'before' and 'after' will be rounded up or down. Previously, this was
supported only when 'before' and 'after' were arrays; string-formatted dates
were run through strtotime(), which rounded them all down (inclusive in the
case of after, non-inclusive in the case of before). Now, we attempt to parse
formats that look like MySQL-formatted date strings, and apply inclusive logic
to them if we recognize them successfully.

Fixes #29908.
string values. Array values support the 'inclusive

git-svn-id: https://develop.svn.wordpress.org/trunk@29936 602fd350-edb4-49c9-b593-d223f7449a82