disable 408 response record any access log with empty request.

In SSL application, Chrome browser may establish extra connections
and doesn't send any data. This will cause a 408 record in the access.log.
Now we could disable this meaningless record with directive 'log_empty_request'.
Merge pull request #195 from yaoweibin/log_empty_request_408

disable 408 response record any access log with empty request.