ensure all saveChanges operations are performed with the read preference set to RP_PRIMARY
Ensure read preferences are restored in the event of an exception being thrown during a save operation
Ensure we only modify the read preference during write when not set to RP_PRIMARY
Modified test to use ->once() instead of ->at()
Added public access to getReadPreference for testing purposes
Merge pull request #4 from talis/XIP3135-EnsureTripodWritesOnlyReadFromPrimary

XIP3135 - Ensure writes only read from primary