Provide content-length header if response has a body

flight/net/Response.php

@@ -191,7 +191,6 @@ class Response {
             $this->headers['Expires'] = gmdate('D, d M Y H:i:s', $expires) . ' GMT';
             $this->headers['Cache-Control'] = 'max-age='.($expires - time());
         }
-
         return $this;
     }
 
@@ -236,6 +235,11 @@ class Response {
             }
         }
 
+        // Send content length
+        if( ($length = strlen($this->body)) > 0) {
+            header("Content-Length: $length");
+        }
+
         return $this;
     }