avoid the need for an nginx core patch

also:
1. when storing an item to cache that already exists dont count it as store error (use a separate counter)
2. check the http status code once the headers are received (no need to read the body in case of error)
3. change the message 'upstream request failed' to be debug only - in case of a real error (not client-disconnected) some previous message will already be printed
4. update the license & copyright text in readme.md