[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

src/qweb/base_directives.ts

@@ -41,6 +41,12 @@ function compileValueNode(value: any, node: Element, qweb: QWeb, ctx: Compilatio
   if (typeof value === "string") {
     exprID = `_${ctx.generateID()}`;
     ctx.addLine(`let ${exprID} = ${ctx.formatExpression(value)};`);
+    if (ctx.isSubTemplate) {
+      ctx.rootContext.shouldDefineUtils = true;
+      ctx.addLine(
+        `${exprID} = ${exprID} instanceof utils.VDomArray ? utils.vDomToString(${exprID}) : ${exprID};`
+      );
+    }
   } else {
     exprID = `scope.${value.id}`;
   }
@@ -238,7 +244,7 @@ QWeb.addDirective({
     if (!subId) {
       subId = QWeb.nextId++;
       qweb.subTemplates[subTemplate] = subId;
-      const subTemplateFn = qweb._compile(subTemplate, nodeTemplate.elem, ctx, true);
+      const subTemplateFn = qweb._compile(subTemplate, nodeTemplate.elem, ctx, true, true);
       QWeb.subTemplates[subId] = subTemplateFn;
     }