Updated test setup to ensure behavior under test is correct

`ResponseTest::testReasonPhraseCanBeEmpty()` was using status 599 as,
prior to #208, no default reason phrase was provided in `Response` for
that code. Now that there is, we need to use a different one. Updated to
555, as it is not currently reserved.

Author: weierophinney

test/ResponseTest.php

@@ -167,7 +167,7 @@ public function testInvalidStatusCodeInConstructor()
 
     public function testReasonPhraseCanBeEmpty()
     {
-        $response = $this->response->withStatus(599);
+        $response = $this->response->withStatus(555);
         $this->assertInternalType('string', $response->getReasonPhrase());
         $this->assertEmpty($response->getReasonPhrase());
     }