Fix Issue #490 (#498)

* Add failing testcase for HTTP 404 response with no reason text.

* Include canonical reason test for HTTP error responses.

* Don't send a reason for unknown status codes.

src/server/error.rs

@@ -21,7 +21,12 @@ impl HttpHandlerTask for ServerError {
             bytes.reserve(helpers::STATUS_LINE_BUF_SIZE + 1);
             helpers::write_status_line(self.0, self.1.as_u16(), bytes);
         }
+        // Convert Status Code to Reason.
+        let reason = self.1.canonical_reason().unwrap_or("");
+        io.buffer().extend_from_slice(reason.as_bytes());
+        // No response body.
         io.buffer().extend_from_slice(b"\r\ncontent-length: 0\r\n");
+        // date header
         io.set_date();
         Ok(Async::Ready(true))
     }