Allow a single instance of a Response object to be set as the next mocked response

tests/Guzzle/Tests/GuzzleTestCase.php

@@ -188,6 +188,12 @@ abstract class GuzzleTestCase extends \PHPUnit_Framework_TestCase
             $that->addMockedRequest($event['request']);
         });
 
+        if ($paths instanceof Response) {
+            // A single response instance has been specified, create an array with that instance
+            // as the only element for the following loop to work as expected
+            $paths = array($paths);
+        }
+
         foreach ((array) $paths as $path) {
             $mock->addResponse($this->getMockResponse($path));
         }