FIX: function php_content_encoding()

- FIX: ( ! ) Warning: Undefined array key "HTTP_ACCEPT_ENCODING" in /xxx/http-headers/http-headers.php on line 583
- FIX: ( ! ) Deprecated: substr_count(): Passing null to parameter #1 ($haystack) of type string is deprecated in /xxx/plugins/http-headers/http-headers.php on line 583

Author: Dareth NHANG