Fix "500 OK" error responses

If the current response already has a status code that corresponds to
the exception that was encountered, it is assumed the developer dealt
with the reason phrase as well, and the reason phrase will be preserved.
If not, the standard message for the resulting HTTP status code will be
used.

src/FinalHandler.php

@@ -120,12 +120,13 @@ public function setOriginalResponse(ResponseInterface $response = null)
      */
     private function handleError($error, RequestInterface $request, ResponseInterface $response)
     {
-        $response = $response->withStatus(
-            Utils::getStatusCode($error, $response),
-            $response->getReasonPhrase()
-        );
-
+        $statusCode = Utils::getStatusCode($error, $response);
+        $reasonPhrase = $response->getStatusCode() === $statusCode
+                      ? $response->getReasonPhrase()
+                      : '';
+        $response = $response->withStatus($statusCode, $reasonPhrase);
         $message = $response->getReasonPhrase() ?: 'Unknown Error';
+
         if (isset($this->options['env']) && $this->options['env'] !== 'production') {
             $message = $this->createDevelopmentErrorMessage($error);
         }

test/FinalHandlerTest.php

@@ -21,6 +21,26 @@
 
 class FinalHandlerTest extends TestCase
 {
+    /**
+     * @var Escaper
+     */
+    private $escaper;
+
+    /**
+     * @var FinalHandler
+     */
+    private $final;
+
+    /**
+     * @var Request
+     */
+    private $request;
+
+    /**
+     * @var Response
+     */
+    private $response;
+
     public function setUp()
     {
         $psrRequest     = new PsrRequest([], [], 'http://example.com/', 'GET', 'php://memory');
@@ -149,13 +169,19 @@ public function testCreates404ResponseWhenNoErrorIsPresent()
         $this->assertEquals(404, $response->getStatusCode());
     }
 
-    public function testErrorResponsePreservesOriginalReasonPhraseIfSet()
+    public function testErrorResponsePreservesResponseReasonPhraseIfStatusCodeMatchesExceptionCode()
     {
         $this->response = $this->response->withStatus(500, 'It broke!');
-        $response = call_user_func($this->final, $this->request, $this->response, new \Exception('foo'));
+        $response = call_user_func($this->final, $this->request, $this->response, new \Exception('foo', 500));
         $this->assertSame($this->response->getReasonPhrase(), $response->getReasonPhrase());
     }
 
+    public function testErrorResponseUsesStandardHttpStatusCodeReasonPhraseIfExceptionCodeCausesStatusCodeToChange()
+    {
+        $response = call_user_func($this->final, $this->request, $this->response, new \Exception('foo', 418));
+        $this->assertSame("I'm a teapot", $response->getReasonPhrase());
+    }
+
     public function test404ResponseIncludesOriginalRequestUri()
     {
         $originalUrl = 'http://local.example.com/bar/foo';