Creation of new logger for "defaultLogger" is counterintuitive.

This line sets up a new logger.

https://github.com/hashicorp/go-retryablehttp/blob/02c1586c8f14be23e7eeb522f1094afbabf45e93/client.go#L53

Using log.Default() would have the same base behaviour.

A user who changes the behaviour of the default logger will not affect the default logger in go-retryablehttp.
An example of this is that the terraform http-backend logs http requests with this line:
https://github.com/hashicorp/go-retryablehttp/blob/02c1586c8f14be23e7eeb522f1094afbabf45e93/client.go#L551
these log lines are unaffected by the terraform logging environment variables.