Our documentation at https://docs.mongodb.com/ruby-driver/master/tutorials/ruby-driver-create-client/#legacy-retryable-writes states:

> If modern retryable writes mechanism is disabled by setting the client option retry_writes: false or by using the retryWrites=false URI option, the driver will utilize the legacy retryable writes mechanism.

It appears that the topology check is performed prior to retryable_writes option check, such that when the driver is in the standalone topology it falls back to legacy retryable writes even when retry_writes=true.

Add tests to be run in single topology that retry_writes=true flag produces no retries by default
Fix driver as necessary