[FIX] web: accept false value in comparison

momentjs `isSame` method considers undefined values but not false
our method _isSameValue does consider false

Correct the error when running web tests
    Deprecation warning: value provided is not in a recognized ISO format

addons/web/static/src/js/fields/basic_fields.js

@@ -417,7 +417,7 @@ var FieldDate = InputField.extend({
             this.datewidget = this._makeDatePicker();
             this.datewidget.on('datetime_changed', this, function () {
                 var value = this._getValue();
-                if ((!value && this.value) || (value && !value.isSame(this.value))) {
+                if ((!value && this.value) || (value && !this._isSameValue(value))) {
                     this._setValue(value);
                 }
             });