Can't set default value via URL query string, for custom date fields

There's not much documentation for it, but for a long time CiviCRM has supported the setting of field values by using URL query parameters. But since 4.7.17 at least (perhaps earlier), this doesn't work for fields with a date picker, though it still works for other fields.

Example URL: http://my.example.com/civicrm/contribute/transact?reset=1&id=5&custom_20=06/06/2017&price_37=20  This should set the custom date field ("custom_20") to a default value of '06/06/2017', and the "other amount" field ("price_37") to a default of "20".  For the price field it works fine, but the date field remains blank. Note this image:

Interesting details:

• The date picker field is not the "real" field, and it turns out the "real" field is just hidden and then populated by the date picker.
• The "real" field actually does get the correct value from the query parameter, as shown in this image, where I've removed the "{display:none}" styling to display the "real" field:

This appears to be the same issue as CRM-18349, in which the date value must be ISO formatted before being set as a default value for crmDatepicker custom field.

PR forthcoming.