Disable gzip compression in debug mode

Dumping raw gzip compressed data on STDERR does not help much, e.g. 

```
reading 107 bytes...
-> "\x1F\x8B\b\x00\x00...
```

output is hardly usable for debugging. Setting `identity` value for `Accept-Encoding` header disables gzip compression of the response, the dumped value is then human readable.