Move dependency-less Page->exit to Core\System::echoResponse

- The method doesn't actually exit
- Fix a bug with header handling in System::echoResponse with numerical key header strings
  - Adding a full-string header with ICanCreateResponses->setHeader was resulting in a wrong header named after the numerical key

src/Module/Media/Attachment/Upload.php

@@ -150,7 +150,7 @@ class Upload extends \Friendica\BaseModule
 			$this->response->addContent($message);
 		}
 
-		$this->page->exit($this->response->generate());
+		System::echoResponse($this->response->generate());
 		System::exit();
 	}
 }

src/Module/Media/Photo/Upload.php

@@ -207,7 +207,7 @@ class Upload extends \Friendica\BaseModule
 			$this->response->addContent($message);
 		}
 
-		$this->page->exit($this->response->generate());
+		System::echoResponse($this->response->generate());
 		System::exit();
 	}
 }