Forms: placeholder field is not designed to take patterns that change date format

The main issue is that the custom placeholder, defined in the Change placeholder field of a Date Picker, is not taken into account in the form.

The issue has been reproduced as below:

1. Installation of Forms Core 2.0.1
2. Installation of Forms Snippets extension 2.0
3. Creation of a Forms form with a Text input and a Date picker fields
4. For the Date picker, modify the Change placeholder to DD/MM/YYYY
5. Publish the Form "test"
6. Create a site with the bootstrap3 Test Templates Set
7. Create a Forms form with the form "test"
8. Try to enter a date with this format: DD/MM/YYYY
   => Date is invalid
9. Try to select a date on the calendar, the format is this one: YYYY-MM-DD
   => The pattern "DD/MM/YYYY" is not taken in account.
   => the accepted date formats are:
   YYYY/MM/DD
   YY/MM/DD
   YYYY-MM-DD
   YY-MM-DD