Blaze: Don't update input elements if their value doesn't need updating

This resolves #2403. Specifically, if you implement some form of
two-way databinding, and you modify an input field in some way
other than adding characters to the end, the insertion point
jumps to the end.

Still need to write a test for this.

packages/blaze/attrs.js

@@ -165,7 +165,8 @@ var BooleanHandler = AttributeHandler.extend({
 
 var ValueHandler = AttributeHandler.extend({
   update: function (element, oldValue, value) {
-    element.value = value;
+    if (value !== element.value)
+      element.value = value;
   }
 });