Fix incorrect class being loaded by lazy model loading.

We need to to correctly handle plugin prefixes in order to load the
correct class.

Refs #3689

src/Controller/Controller.php

@@ -311,12 +311,9 @@ public function addComponent($name, array $config = []) {
  */
 	public function __get($name) {
 		if (strpos($this->modelClass, $name) !== false) {
-			list($plugin, $class) = pluginSplit($name, true);
-			if (!$plugin) {
-				$plugin = $this->plugin ? $this->plugin . '.' : null;
-			}
-			$this->loadModel($plugin . $this->modelClass);
-			return $this->{$this->modelClass};
+			list($plugin, $class) = pluginSplit($this->modelClass, true);
+			$this->loadModel($plugin . $class);
+			return $this->{$class};
 		}
 		return false;
 	}

tests/TestCase/Controller/ControllerTest.php

@@ -312,7 +312,7 @@ public function testConstructSetModelClass() {
 		$request->params['plugin'] = 'TestPlugin';
 		$controller = new \TestPlugin\Controller\Admin\CommentsController($request, $response);
 		$this->assertEquals('TestPlugin.Comments', $controller->modelClass);
-		$this->assertInstanceOf('Cake\ORM\Table', $controller->Comments);
+		$this->assertInstanceOf('TestPlugin\Model\Table\CommentsTable', $controller->Comments);
 	}
 
 /**

tests/test_app/Plugin/TestPlugin/src/Model/Table/CommentsTable.php

@@ -0,0 +1,25 @@
+<?php
+/**
+ * CakePHP :  Rapid Development Framework (http://cakephp.org)
+ * Copyright (c) Cake Software Foundation, Inc. (http://cakefoundation.org)
+ *
+ * Licensed under The MIT License
+ * For full copyright and license information, please see the LICENSE.txt
+ * Redistributions of files must retain the above copyright notice.
+ *
+ * @copyright     Copyright (c) Cake Software Foundation, Inc. (http://cakefoundation.org)
+ * @link          http://cakefoundation.org/projects/info/cakephp CakePHP Project
+ * @since         3.0.0
+ * @license       http://www.opensource.org/licenses/mit-license.php MIT License
+ */
+namespace TestPlugin\Model\Table;
+
+use Cake\ORM\Table;
+
+/**
+ * Class CommentsTable
+ *
+ */
+class CommentsTable extends Table {
+
+}