[FIX] component: fix scoping issue in nested loops

The templates contained in a slot have to be rendered with the current
scope, but during the rendering of the sub component, which happens
later (after the willStart for the sub component). Therefore, we need to
save the scope that should be used for the slots, so we can access the
proper variables. This was done using an Object.assign() statement,
which is actually only a shallowclone for the own properties.

In this commit, we properly copy all the properties for the current
scope, even those contained in the prototype.

closes #855

tests/component/slots.test.ts

@@ -325,6 +325,70 @@ describe("t-slot directive", () => {
     expect(QWeb.slots["1_default"].toString()).toMatchSnapshot();
   });
 
+  test("slots in t-foreach in t-foreach", async () => {
+    class Child extends Component {
+      static template = xml`
+        <div><t t-slot="default" /></div>
+      `;
+    }
+
+    class App extends Component {
+      static template = xml`
+        <div>
+          <t t-foreach="tree" t-as="node1" t-key="node1.key">
+            <div t-esc="node1.value" />
+            <ul>
+              <t t-foreach="node1.nodes" t-as="node2" t-key="node2.key">
+                <Child>
+                  <li t-esc="node1.value" />
+                </Child>
+              </t>
+            </ul>
+          </t>
+        </div>`;
+
+      static components = { Child };
+
+      tree = [
+        {
+          key: "a",
+          value: "A",
+          nodes: [
+            {
+              key: "1",
+              value: "A-1",
+            },
+            {
+              key: "2",
+              value: "A-2",
+            },
+          ],
+        },
+        {
+          key: "b",
+          value: "B",
+          nodes: [
+            {
+              key: "1",
+              value: "B-1",
+            },
+            {
+              key: "2",
+              value: "B-2",
+            },
+          ],
+        },
+      ];
+    }
+
+    await mount(App, { target: fixture });
+
+    expect(fixture.innerHTML).toBe(
+      "<div><div>A</div><ul><div><li>A</li></div><div><li>A</li></div></ul><div>B</div><ul><div><li>B</li></div><div><li>B</li></div></ul></div>"
+    );
+    expect(env.qweb.templates[App.template].fn.toString()).toMatchSnapshot();
+  });
+
   test("refs are properly bound in slots", async () => {
     class Dialog extends Component {
       static template = xml`<span><t t-slot="footer"/></span>`;