Primitives now use valueOf in shouldGetter

This allows the should syntax to be more resilient when
dealing with modified primitive constructors which
may occur more frequently with ES6 to ES5 transpilation.

lib/chai/interface/should.js

@@ -10,10 +10,8 @@ module.exports = function (chai, util) {
   function loadShould () {
     // explicitly define this method as function as to have it's name to include as `ssfi`
     function shouldGetter() {
-      if (this instanceof String || this instanceof Number) {
-        return new Assertion(this.constructor(this), null, shouldGetter);
-      } else if (this instanceof Boolean) {
-        return new Assertion(this == true, null, shouldGetter);
+      if (this instanceof String || this instanceof Number || this instanceof Boolean ) {
+        return new Assertion(this.valueOf(), null, shouldGetter);
       }
       return new Assertion(this, null, shouldGetter);
     }