Update code style

Author: adamquaile

src/Client.php

@@ -119,16 +119,19 @@ public function on(RequestMatcher $requestMatcher, $result)
         switch (true) {
             case is_callable($result):
                 $callable = $result;
+
                 break;
             case $result instanceof ResponseInterface:
                 $callable = function () use ($result) {
                     return $result;
                 };
+
                 break;
             case $result instanceof \Exception:
                 $callable = function () use ($result) {
                     throw $result;
                 };
+
                 break;
             default:
                 throw new \InvalidArgumentException('Result must be either a response, an exception, or a callable');