Support changing max_retries

[fschwahn]

net/http (and by extension open-uri) by default retries GET requests one time (max_retries = 1). It would be great if this setting could be altered through open-uri. It is already possible to change timeouts, and if it would be possible to disable retries I could ensure that the request either suceeds or fails in a predictable timeframe.

Maybe a more general option would make sense which could allow us to pass any option through to Net::Http?