FSBucket's read_preference method is currently written as follows:

      def read_preference
        @read_preference ||= options[:read] || database.read_preference
      end

options[:read] is a Hash, database.read_preference is a BSON::Document. Thus depending on what value is passed into constructor of FSBucket, the value returned from read_preference may change type.

The docstring is also incorrect on the constructor claiming :read option is a string (it is actually a Hash), and on read_preference method claiming to return the preference as a ServerSelector object (the return value is either a Hash or a BSON::Document).

For consistency with client and database objects let's change FSBucket to return read preference always as a BSON::Document. Unfortunately this is an incompatible change.

The same issue affects Grid::FSBucket::Stream::Read.