Do() should checks about CheckRetry value before calling it

I have integrated go-retryablehttp but went into an issue on segfault when Do() function calls "CheckRetry".

That was because my code create the Client struct itself using:
`hc := &http.Client{}`

So all extra members of the Client object were not defined (like CheckRetry).

Switching to:
`hc := retryablehttp.NewClient()`
Fixed the issue.

So, I think the library should check "CheckRetry" value before calling it.