Bump to v4.0.0.

internal/assignMergeValue.js

@@ -0,0 +1,22 @@
+define(['../eq'], function(eq) {
+
+  /** Used as a safe reference for `undefined` in pre-ES5 environments. */
+  var undefined;
+
+  /**
+   * This function is like `assignValue` except that it doesn't assign `undefined` values.
+   *
+   * @private
+   * @param {Object} object The object to modify.
+   * @param {string} key The key of the property to assign.
+   * @param {*} value The value to assign.
+   */
+  function assignMergeValue(object, key, value) {
+    if ((value !== undefined && !eq(object[key], value)) ||
+        (typeof key == 'number' && value === undefined && !(key in object))) {
+      object[key] = value;
+    }
+  }
+
+  return assignMergeValue;
+});