Bump to v4.17.3.

_customDefaultsAssignIn.js

@@ -0,0 +1,33 @@
+define(['./eq'], function(eq) {
+
+  /** Used as a safe reference for `undefined` in pre-ES5 environments. */
+  var undefined;
+
+  /** Used for built-in method references. */
+  var objectProto = Object.prototype;
+
+  /** Used to check objects for own properties. */
+  var hasOwnProperty = objectProto.hasOwnProperty;
+
+  /**
+   * Used by `_.defaults` to customize its `_.assignIn` use to assign properties
+   * of source objects to the destination object for all destination properties
+   * that resolve to `undefined`.
+   *
+   * @private
+   * @param {*} objValue The destination value.
+   * @param {*} srcValue The source value.
+   * @param {string} key The key of the property to assign.
+   * @param {Object} object The parent object of `objValue`.
+   * @returns {*} Returns the value to assign.
+   */
+  function customDefaultsAssignIn(objValue, srcValue, key, object) {
+    if (objValue === undefined ||
+        (eq(objValue, objectProto[key]) && !hasOwnProperty.call(object, key))) {
+      return srcValue;
+    }
+    return objValue;
+  }
+
+  return customDefaultsAssignIn;
+});