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