fix: use default retry hooks for zero-value Client

[happysnaker]

Summary

• fall back to DefaultRetryPolicy when Client.CheckRetry is nil
• fall back to DefaultBackoff when Client.Backoff is nil
• add regression coverage showing a manually constructed zero-value Client no longer panics and still retries

Why

NewClient() initializes these hooks, but the Client type is public and issue #108 reports a real panic when callers construct &retryablehttp.Client{} directly. This change keeps that usage from crashing in Do() while preserving the existing behavior for fully initialized clients.

Closes #108