Only convert numeric read preference types (driver <=1.3.2)

In driver version 1.3.3+, getReadPreference() will return a type in the format accepted by setReadPreference(), which obviates the need for conversion. See: https://jira.mongodb.org/browse/PHP-638
Only convert tag sets if necessary for setReadPreference()

This ensures the conversion function properly does nothing in driver version 1.3.3+. Adding the NOP logic in the calling context would lead to duplicated code, so we allow the utility function to be invoked even if there is nothing to do.

See: https://jira.mongodb.org/browse/PHP-638