[FIX] qweb: enclose interpolated expr in parentheses

Since rev 53e53e9b,
 <div class="o_app" t-attf-class="{{ true ? 'o_extra' : '' }}"/>
produced
 <div class="o_extra"/>
whereas it should have produced
 <div class="o_app o_extra"/>

This is because there were missing parentheses around the string
returned by `interpolate`, which produced instructions like

 var _1 = 'o_app ' + true ? 'o_extra' : '';

Fixes #132

src/qweb_core.ts

@@ -754,19 +754,20 @@ export class Context {
 
   /**
    * Perform string interpolation on the given string. Note that if the whole
-   * string is an expression, it simply returns it (formatted).
+   * string is an expression, it simply returns it (formatted and enclosed in
+   * parentheses).
    * For instance:
    *   'Hello {{x}}!' -> `Hello ${x}`
-   *   '{{x}}' -> x
+   *   '{{x ? 'a': 'b'}}' -> (x ? 'a' : 'b')
    */
   interpolate(s: string): string {
     let matches = s.match(/\{\{.*?\}\}/g);
     if (matches && matches[0].length === s.length) {
-      return this.formatExpression(s.slice(2, -2));
+      return `(${this.formatExpression(s.slice(2, -2))})`;
     }
     matches = s.match(/\#\{.*?\}/g);
     if (matches && matches[0].length === s.length) {
-      return this.formatExpression(s.slice(2, -1));
+      return `(${this.formatExpression(s.slice(2, -1))})`;
     }
 
     let formatter = expr => "${" + this.formatExpression(expr) + "}";