txn_read_pref takes `@client.read_preference` and pastes it into the `$readPreference` in the document sent to the server. The problem is that the former is using underscore naming and the latter camel case. The result is server error like so:

[2018/07/05 13:34:49.695] Mongo::Error::OperationFailure:
[2018/07/05 13:34:49.695] Could not parse $readPreference mode 'primary_preferred'. Only the modes 'primary', 'primaryPreferred', secondary', 'secondaryPreferred', and 'nearest' are supported. (9)