Dates for new or updated posts should honor date parameters passed

There is a bug in the logic of WP_JSON_Posts->insert_post() where even if the `$data['date']`, `$data['date_gmt']`, `$data['modified']`, or  `$data['modified_gmt']` are not empty the values are populated with the current time in GMT.

This appears to be a blocker for the 1.0 release, if this bug is confirmed by @rmccue.

See current logic here: See current logic here: https://github.com/WP-API/WP-API/blob/master/lib/class-wp-json-posts.php#L1185