[FIX] web: FieldSelection on m2o: unset value

When the value of a many2one field with widget selection was unset,
it's value wasn't actually set to false in the BasicModel. It was
set to a virtual id like 'virtual_294' instead. This caused a
traceback when this value was sent to the server, obviously.

This was for example the case of the Register Payment form view
(accessible from a customer invoice), when the user selected a
payment journal, and then unselected it (the virtual id was then
sent to the onchange RPC).

addons/web/static/src/js/views/basic/basic_model.js

@@ -1001,7 +1001,7 @@ var BasicModel = AbstractModel.extend({
      */
     _applyMany2OneChange: function (record, fieldName, data) {
         var self = this;
-        if (!data) {
+        if (!data || data.id === false) {
             record._changes[fieldName] = false;
             return $.when();
         }

addons/web/static/tests/fields/relational_fields_tests.js

@@ -5797,6 +5797,35 @@ QUnit.module('relational_fields', {
         form.destroy();
     });
 
+    QUnit.test('unset selection on a many2one field', function (assert) {
+        assert.expect(1);
+
+        var form = createView({
+            View: FormView,
+            model: 'partner',
+            data: this.data,
+            arch: '<form string="Partners">' +
+                        '<field name="trululu" widget="selection"/>' +
+                '</form>',
+            mockRPC: function (route, args) {
+                if (args.method === 'write') {
+                    assert.strictEqual(args.args[1].trululu, false,
+                        "should send 'false' as trululu value");
+                }
+                return this._super.apply(this, arguments);
+            },
+            res_id: 1,
+            viewOptions: {
+                mode: 'edit',
+            },
+        });
+
+        form.$('select').val("false").trigger('change');
+        form.$buttons.find('.o_form_button_save').click();
+
+        form.destroy();
+    });
+
     QUnit.module('FieldMany2ManyTags');
 
     QUnit.test('fieldmany2many tags: rendering and edition', function (assert) {