[FIX] qweb: fix issue variables set in body of t-call

The body of a t-call directive may be used to define private variables
to the sub template call.

However, the code that handles t-call worked like this:

- compile sub template if necessary
- then compile body of t-call to extract variables

This means that the variables defined in the t-call body were not yet
processed and available in the context.  Because of that, when the call
to t-esc is done, there is not internal qweb var, and the code simply
outputs a scope['varname'], which is in our case a VDOMArray, so it is
displayed as [object object]

What this fix does is changing the way t-esc works: if we are in the
context of a sub template, then it assumes that any outside variable may
or may not be a VDomArray, so it needs to check and eventually convert
it to a string, if necessary.

closes #719

tests/qweb/qweb.test.ts

@@ -1072,6 +1072,21 @@ describe("t-call (template calling", () => {
     expect(renderToString(qweb1, "main")).toBe("<div><span>ok</span></div>");
     expect(renderToString(qweb2, "main")).toBe("<div><span>ok</span></div>");
   });
+
+  test("t-call with t-set inside and body text content", () => {
+    qweb.addTemplate("sub", `<p><t t-esc="val"/></p>`);
+    qweb.addTemplate(
+      "main",
+      `
+      <div>
+        <t t-call="sub">
+          <t t-set="val">yip yip</t>
+        </t>
+      </div>`
+    );
+    const expected = "<div><p>yip yip</p></div>";
+    expect(renderToString(qweb, "main")).toBe(expected);
+  });
 });
 
 describe("foreach", () => {