Concise-ification has broken default errors

There is a bug in the default error lookup:

    exception_types_to_rescue = Array(options[:rescue]) || [StandardError]

caused by this commit: https://github.com/ooyala/retries/commit/4447c4c988667b6c9c84a34d8f87157b3123adbf

When no rescue option is supplied, it will never catch any error at all, since:

     Array(nil) || [StandardError]   # => [], not [StandardError] as hoped!

Resolution: 

1) Put the nil check in the Array creator
2) Open beer

Cheers!