Fix / being handled incorrect by referer()

Backport changes in #4987 to 2.x. This solves issues with duplicate base
directories when redirecting back to '/'

Fixes #4812

lib/Cake/Controller/Controller.php

@@ -974,7 +974,7 @@ class Controller extends Object implements CakeEventListener {
 		}
 
 		$referer = $this->request->referer($local);
-		if ($referer === '/' && $default) {
+		if ($referer === '/' && $default && $default !== $referer) {
 			return Router::url($default, !$local);
 		}
 		return $referer;

lib/Cake/Test/Case/Controller/ControllerTest.php

@@ -1031,6 +1031,30 @@ class ControllerTest extends CakeTestCase {
 		$this->assertEquals('/', $result);
 	}
 
+/**
+ * Test that the referer is not absolute if it is '/'.
+ *
+ * This avoids the base path being applied twice on string urls.
+ *
+ * @return void
+ */
+	public function testRefererSlash() {
+		$request = $this->getMock('CakeRequest', array('referer'));
+		$request->base = '/base';
+		$request->expects($this->any())
+			->method('referer')
+			->will($this->returnValue('/'));
+		Router::setRequestInfo($request);
+
+		$controller = new Controller($request);
+		$result = $controller->referer('/', true);
+		$this->assertEquals('/', $result);
+
+		$controller = new Controller($request);
+		$result = $controller->referer('/some/path', true);
+		$this->assertEquals('/base/some/path', $result);
+	}
+
 /**
  * testSetAction method
  *