Fix header key case in fillKey

The problem is that [`headers`](https://github.com/etehtsea/torquebox/blob/fc253afd8268de1827bebb7e4808ecf1e9be47e8/web/ext/wunderboss-rack/org/projectodd/wunderboss/rack/RackEnvironmentHash.java#L137) contains camelcased keys, but `rubyKey` is upcased.

```
[5] pry(#<Rack::Builder>)> env['HTTP_USER_AGENT']
=> nil
[6] pry(#<Rack::Builder>)> env.to_s
=> "{\"SERVER_NAME\"=>\"0.0.0.0\", \"HTTP_ACCEPT\"=>\"*/*\", \"rack.multithread\"=>true, \"rack.url_scheme\"=>\"http\", \"rack.multiprocess\"=>true, \"rack.input\"=>#<WunderBoss::RackChannel:0x74fd7616>, \"rack.errors\"=>#<IO:fd 562>, \"CONTENT_TYPE\"=>\"null\", \"REQUEST_URI\"=>\"/\", \"rack.version\"=>[1, 1], \"SERVER_PORT\"=>\"9292\", \"REQUEST_METHOD\"=>\"GET\", \"REMOTE_ADDR\"=>\"127.0.0.1\", \"rack.run_once\"=>false, \"PATH_INFO\"=>\"/\", \"SCRIPT_NAME\"=>\"\", \"QUERY_STRING\"=>\"\", \"HTTP_USER_AGENT\"=>\"curl/7.30.0\", \"HTTP_HOST\"=>\"0.0.0.0:9292\"}"
```

P.S. I'm not sure about the best way to fix this issue. I'm not a Java-guy to be honest.