Currently mongo-ruby-driver does not perform any validation on read option to Client constructor. In particular the following is silently accepted:

Mongo::Client.new(['127.0.0.1:27017'], read: :nearest) 

... but the read setting is ignored since it is treated as a hash.

Suggested changes are:

1. Reject read values which are strings and symbols. A stricter check would require the value to be a Hash but this can break clients which pass class instances that are not derived from Hash but implement key access ([]).
2. If the read option value is a hash or similar object having the :mode key, require that the mode is one of recognized modes.