Prevent 'Uncaught TypeError: explode(): Argument #2 ($string) must be of type string, null given' (#47)

* fix: prevent 'explode' error when responseContentType is null.
* always use casting to string to hit more cases.

Author: Fahl-Design

src/Codeception/Lib/InnerBrowser.php

@@ -97,7 +97,7 @@ public function _failed(TestInterface $test, $fail)
             $internalResponse = false;
         }
 
-        $responseContentType = $internalResponse ? $internalResponse->getHeader('content-type') : '';
+        $responseContentType = $internalResponse ? (string) $internalResponse->getHeader('content-type') : '';
         [$responseMimeType] = explode(';', $responseContentType);
 
         $extension = $extensions[$responseMimeType] ?? 'html';