avoid duplicate requests to the cache proxy. 

fix #125
it should indeed be case-insensitive for header names (it is technically currently as it ignores the name entirely), not for header values. The method should probably be case-insensitive.

So here is my proposal:

```php
$headers = array_change_key_case($headers, CASE_LOWER);
hsort($headers);

return md5(strtoupper($method)."\n".$url."\n".var_export($headers, true));
```

an alternative to ``var_export`` could be ``json_encode`` but it does not change much things here