Allow null reason phrase

src/Message/Response.php

@@ -125,8 +125,6 @@ class Response extends Message implements ResponseInterface
             }
             if (isset($reasonPhraseLookup[$code])) {
                 $reasonPhrase = $reasonPhraseLookup[$code];
-            } else {
-                $reasonPhrase = "Unknown";
             }
         }
         $response->reasonPhrase = $reasonPhrase;

test/tests/unit/Message/ResponseTest.php

@@ -75,7 +75,7 @@ class ResponseTest extends \PHPUnit_Framework_TestCase
             [503, null, "Service Unavailable"],
             [504, null, "Gateway Timeout"],
             [505, null, "HTTP Version Not Supported"],
-            [598, null, "Unknown"],
+            [598, null, null],
             [599, "Nonstandard", "Nonstandard"]
         ];
     }