RUBY-2184: Make $readPreference behave as specified

[egiurleo]

From the server selection spec:

Therefore, when sending queries to a mongos, the following rules apply:

• For mode 'primary', drivers MUST NOT set the slaveOK wire protocol flag and MUST NOT use $readPreference
• For mode 'secondary', drivers MUST set the slaveOK wire protocol flag and MUST also use $readPreference
• For mode 'primaryPreferred', drivers MUST set the slaveOK wire protocol flag and MUST also use $readPreference
• For mode 'secondaryPreferred', drivers MUST set the slaveOK wire protocol flag. If the read preference contains a non-empty tag_sets parameter, maxStalenessSeconds is a positive integer, or the hedge parameter is non-empty, drivers MUST use $readPreference; otherwise, drivers MUST NOT use $readPreference
• For mode 'nearest', drivers MUST set the slaveOK wire protocol flag and MUST also use $readPreference

Our read preference op message tests did not test for this behavior, so I have updated the tests and modified the existing code. The actual work for RUBY-2184 will be completed on top of this work.