Add _get_user_input_value on Field, in order to improve compatibility between DateTimeField and ProxyField.
This method get in the REQUEST the input data entered by the user.


git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@13914 20353a03-c40f-0410-a6d1-a30d3c3de9de