fix node index displacement by null/undefined nodes

tests/mithril-tests.js

@@ -313,6 +313,14 @@ function testMithril(mock) {
 		var elementAfter = root.childNodes[0]
 		return elementBefore !== elementAfter
 	})
+	test(function() {
+		//https://github.com/lhorie/mithril.js/issues/56
+		var root = mock.document.createElement("div")
+		m.render(root, [null, "foo"])
+		m.render(root, ["bar"])
+		console.log(root.childNodes)
+		return root.childNodes.length == 1
+	})
 	//end m.render
 	
 	//m.redraw

tests/mock.js

@@ -12,9 +12,12 @@ mock.window = new function() {
 			replaceChild: window.document.replaceChild,
 			insertBefore: function(node, reference) {
 				node.parentNode = this
-				var index = this.childNodes.indexOf(reference)
-				if (index < 0) this.childNodes.push(node)
-				else this.childNodes.splice(index, 0, node)
+				var referenceIndex = this.childNodes.indexOf(reference)
+				if (referenceIndex < 0) this.childNodes.push(node)
+				else {
+					var index = this.childNodes.indexOf(node)
+					this.childNodes.splice(referenceIndex, index < 0 ? 0 : 1, node)
+				}
 			},
 			insertAdjacentHTML: function(position, html) {
 				//todo: accept markup
@@ -54,6 +57,8 @@ mock.window = new function() {
 		oldChild.parentNode = null
 	}
 	window.document.appendChild = function(child) {
+		var index = this.childNodes.indexOf(child)
+		if (index > -1) this.childNodes.splice(index, 1)
 		this.childNodes.push(child)
 		child.parentNode = this
 	}