Return cached pages only if request method is GET or HEAD

[tobiasfabian]

If the request method is POST, PUT, or similar, no cached pages should be returned.

The following could be added to the index.php (compare to $page->isCacheable())

(function /* staticache */ () {
	// check if request method is not GET or HEAD
	if (in_array($_SERVER['REQUEST_METHOD'], ['GET', 'HEAD']) === false) {
		return;
	}

	// …