Fix up tests.

src/Console/Shell.php

@@ -671,7 +671,7 @@ class Shell
      */
     public function warn($message = null, $newlines = 1)
     {
-        return $this->err('<warning>' . $message . '</warning>', $newlines);
+        return $this->_io->err('<warning>' . $message . '</warning>', $newlines);
     }
 
     /**

tests/TestCase/Console/ShellTest.php

@@ -334,7 +334,7 @@ class ShellTest extends TestCase
     public function testWarn()
     {
         $this->io->expects($this->once())
-            ->method('warn')
+            ->method('err')
             ->with('<warning>Just a test</warning>', 1);
 
         $this->Shell->warn('Just a test');