Non-A+ behavior

I played a bit with `MPromise` and found bug in the way it handles `onResolve` and `onReject`.

Consider the following example:

    var Promise = require('mpromise');

    var p = new Promise;

    p.then(console.log, console.log);

    p.fulfill('foo'); // foo
    p.fulfill('bar'); // bar
    p.reject('baz'); // baz

    p.then(console.log, console.log); // nothing

But it is against A+ spec. Ones promise if fulfilled or rejected it should preserve its state.