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/App/Page.php

@@ -401,36 +401,6 @@ class Page implements ArrayAccess
 		$this->footerScripts[] = trim($url, '/');
 	}
 
-	/**
-	 * Directly exit with the current response (include setting all headers)
-	 *
-	 * @param ResponseInterface $response
-	 */
-	public function exit(ResponseInterface $response)
-	{
-		header(sprintf("HTTP/%s %s %s",
-			$response->getProtocolVersion(),
-			$response->getStatusCode(),
-			$response->getReasonPhrase())
-		);
-
-		foreach ($response->getHeaders() as $key => $header) {
-			if (is_array($header)) {
-				$header_str = implode(',', $header);
-			} else {
-				$header_str = $header;
-			}
-
-			if (empty($key)) {
-				header($header_str);
-			} else {
-				header("$key: $header_str");
-			}
-		}
-
-		echo $response->getBody();
-	}
-
 	/**
 	 * Executes the creation of the current page and prints it to the screen
 	 *