Fix fetch and add unit test.

[ptoomey3]

Current behavior of fetch is incorrect if the default value passed is a hash. This occurs because of the use of convert_hashes_to_parameters. If the value passed to convert_hashes_to_parameters is a hash, this value will be converted to a Parameters hash and stored at self[key]. This seems inconsistent with how default values works with regular hashes.

Also, looks like this file had a bunch of trailing whitespace...if you want a clean blame log I can resubmit the PR without the whitespace cleanup.

[ptoomey3]

It loos like this issue may be accidentally handled by rails/rails#11695. That PR was originally referenced from #156. It looks like the proposed PR fixes both the Exception issue as well as the convert_hashes_to_parameters issue. I can submit a reasonable unit test once that PR is merged.