refactor($interpolate): remove some unnecessary logic, add boolean stringify shortcut

Author: jbedard

src/ng/interpolate.js

@@ -249,15 +249,12 @@ function $InterpolateProvider() {
           }
           switch (typeof value) {
             case 'string':
-              break;
             case 'number':
-              value = '' + value;
-              break;
+            case 'boolean':
+              return value;
             default:
-              value = toJson(value);
+              return toJson(value);
           }
-
-          return value;
         };
 
         return extend(function interpolationFn(context) {