Add max_retries options to configure Net::HTTP idempotent request retry

[dwiyanr]

After the release of ruby 2.5.0, Net::HTTP by default retry idempotent request in case of failure ref. I think this can lead to unexpected behavior which needs to be configurable.
As far as I see, we cannot configure this in the current version of net-http-persistent. My suggestion is to add options for max_retries to pass to Net::HTTP

[drbrain]

Net::HTTP::Persistent no longer automatically retries requests as all versions of ruby it supports retry the requests automatically.

The option to enable retries will be removed in a future release, #100 is a PR to do this that is not yet merged.