Accommodate nil server setting via ActiveSupport

This wasn't actually working before :). `@servers` would end up being `[nil]`

All tests pass if i had changed `addresses = addresses.flatten` to `addresses = addresses.flatten.compact`, but I felt it was more conservative to do it after the options parsing was done, just in case there were edge cases not covered in specs.

I threw in the server check in the "normalize options" test for good measure