Fixed invalid header value in test

It is not possible that header value will be true for HTTPS.
It is `on`/`On`/`ON` when it's enabled, and `off` (apache) or empty string
(nginx) otherwise.

test/ServerRequestFactoryTest.php

@@ -295,7 +295,7 @@ public function testMarshalUriDetectsHttpsSchemeFromServerValue($param)
         $request = $request->withHeader('Host', 'example.com');
 
         $server  = [
-            $param => true,
+            $param => 'on',
         ];
 
         $uri = marshalUriFromSapi($server, $request->getHeaders());