Default constructor for subclass now returns returned value from parent

[rymohr]

This simple change makes it easy to implement identity maps (see included tests for example). Been needing something like this for my work with Backbone recently.

I left external constructors alone for now, as I don't fully understand their purpose as defined by the current test suite (external constructors return their own instances but are expected to behave as the calling class?):

  #1182: external constructors with bound functions 
  TypeError: Cannot call method 'call' of undefined
    at Function.<anonymous> (/Volumes/HD2/devel/opensource/coffee-script/test/classes.coffee:1042:30)
    at /Volumes/HD2/devel/opensource/coffee-script/Cakefile:203:12
    at Object.<anonymous> (/Volumes/HD2/devel/opensource/coffee-script/test/classes.coffee:1007:3)
    at Object.<anonymous> (/Volumes/HD2/devel/opensource/coffee-script/test/classes.coffee:1232:4)
    at Module._compile (module.js:432:26)
    at Object.run (/Volumes/HD2/devel/opensource/coffee-script/lib/coffee-script/coffee-script.js:66:25)
    at /Volumes/HD2/devel/opensource/coffee-script/Cakefile:271:22
    at Object.action (/Volumes/HD2/devel/opensource/coffee-script/Cakefile:285:12)
    at /Volumes/HD2/devel/opensource/coffee-script/lib/coffee-script/cake.js:39:26
    at Object.run (/Volumes/HD2/devel/opensource/coffee-script/lib/coffee-script/cake.js:62:21) 
  test/classes.js: line unknown, column unknown 
  function () {
    var A, B, fn, _class;
    fn = function() {
      return {
        one: 1
      };
    };
    B = (function() {

      function B() {
        var instance = this;
        return instance;
      }

      return B;

    })();
    A = (function() {

      function A() {
        this.method = __bind(this.method, this);
        var instance = this;
        instance = _class.apply(this, arguments);
        return instance;
      }

      _class = fn;

      A.prototype.method = function() {
        return this instanceof A;
      };

      return A;

    })();
    return ok((new A).method.call(new B));
  }

[michaelficarra]

Duplicate of #1970.