refactor(createIndex): simplify argument management for createIndex

Collection.prototype.createIndex, and Db.prototype.createIndex both
had rather complicated argument management code to handle legacy
forms of the method. Since we have committed to an explicit
signature, we can now simplfy this code as well.

lib/collection.js

@@ -1512,12 +1512,8 @@ define.classMethod('isCapped', { callback: true, promise: true });
  * @return {Promise} returns Promise if no callback passed
  */
 Collection.prototype.createIndex = function(fieldOrSpec, options, callback) {
-  var args = Array.prototype.slice.call(arguments, 1);
-  callback = typeof args[args.length - 1] === 'function' ? args.pop() : undefined;
-
-  options = args.length ? args.shift() || {} : {};
-  options = typeof callback === 'function' ? options : callback;
-  options = options == null ? {} : options;
+  if (typeof options === 'function') (callback = options), (options = {});
+  options = options || {};
 
   return executeOperation(this.s.topology, createIndex, [this, fieldOrSpec, options, callback]);
 };

lib/db.js

@@ -1043,13 +1043,8 @@ define.classMethod('executeDbAdminCommand', { callback: true, promise: true });
  * @return {Promise} returns Promise if no callback passed
  */
 Db.prototype.createIndex = function(name, fieldOrSpec, options, callback) {
-  var args = Array.prototype.slice.call(arguments, 2);
-  callback = typeof args[args.length - 1] === 'function' ? args.pop() : undefined;
-  options = args.length ? args.shift() || {} : {};
-  options = typeof callback === 'function' ? options : callback;
-  options = options == null ? {} : options;
-  // Shallow clone the options
-  options = shallowClone(options);
+  if (typeof options === 'function') (callback = options), (options = {});
+  options = options ? shallowClone(options) : {};
 
   return executeOperation(this.s.topology, createIndex, [
     this,