[javascript/en] Re-add note about truthiness of wrapped primitives

Previous incarnations of this section had `Number(0)` instead of
`new Number(0)`, which actually returns `0` due to the absence of the
`new` keyword; this commit re-adds that section and better still, makes
it actually correct!

javascript.html.markdown

@@ -507,6 +507,10 @@ myNumber === myNumberObj; // = false
 if (0){
     // This code won't execute, because 0 is falsy.
 }
+if (new Number(0)){
+   // This code will execute, because wrapped numbers are objects, and objects
+   // are always truthy.
+}
 
 // However, the wrapper objects and the regular builtins share a prototype, so
 // you can actually add functionality to a string, for instance.