Fix map reduce with a read preference (#4545)

Author: p-mongo

lib/mongoid/contextual/map_reduce.rb

@@ -165,7 +165,7 @@ def output
       def raw
         validate_out!
         cmd = command
-        opts = { read: cmd.delete(:read).options } if cmd[:read]
+        opts = { read: cmd.delete(:read) } if cmd[:read]
         @map_reduce.database.command(cmd, (opts || {}).merge(session: _session)).first
       end
       alias :results :raw