BST - remove root node fix BUG

[JakeHP]

My fix for #46 explained here #47 was a 'woosh'. If root has children, with that change in place, they just become orphans and are lost in the void (rip).

if (node._parent !== null) {
  if (node._left) {
    this._replaceChild(node._parent, node, node._left);
  } else if (node._right) {
    this._replaceChild(node._parent, node, node._right);
  } else {
    this._replaceChild(node._parent, node, null);
  }
}else {
  this._root = null; // all children of that node lost in void
}

So I've removed the code from #47 and have a new fix that isn't stupid, tested, and works.

  exports.BinaryTree.prototype._replaceChild =
   function (parent, oldChild, newChild) {
    if (!parent) {
      this._root = newChild;
      if(this._root !== null){ //ADDED
        this._root._parent = null;
      } //ADDED
    } else {
      if (parent._left === oldChild) {
        parent._left = newChild;
      } else {
        parent._right = newChild;
      }
      if (newChild) {
        newChild._parent = parent;
      }
    }
  };

The new test - root removal with children:

  it('should remove root and replace with valid child (1)', function () {
    var bTree = new BinaryTree();
    bTree.insert(15);
    bTree.insert(30);
    bTree.insert(45);
    var node = bTree.find(15);
    bTree.remove(node);
    expect(bTree._root.value).toBe(30);
  });

will submit PR after #51 is handled (on same branch again 👯)

I'm going to be adding a bunch of tests to prevent these joys

[JakeHP]

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