[Michael McQuade (giraffesyo)]

A generated client could retry a redirect its policy had refused. The retry recognized a refusal only by an error message starting "stopped after", or by a response returned with the error. A caller's own CheckRedirect worded otherwise, behind a middleware that drops the response, was sent again.

What changes:

• redirect.go gains an exported RedirectRefusedError{Err error} with Unwrap. Its message is the refusal's own, so error text is unchanged, and errors.Is/errors.As on the original error still work.
• SafeRedirects always returns a copy of the client whose CheckRedirect wraps the effective policy: the caller's own when set, otherwise the generated CheckRedirect. The caller's policy still decides every redirect. It is wrapped, never replaced. http.ErrUseLastResponse passes through unwrapped, because net/http compares it with ==.
• isRetryableNetworkError declines a *RedirectRefusedError and drops the "stopped after" prefix check. send also declines when Client.Do returns a response along with the error, which is net/http's signal for a CheckRedirect refusal.
• RedirectRefusedError is added to the reserved identifiers.

Tests in e2e_redirect_test.go cover:

• a caller's refusal with any message, or wrapping ErrInsecureRedirect, is tried once;
• a caller's refusal behind a middleware that drops the response is tried once and comes back as a *RedirectRefusedError wrapping the caller's error;
• the client's own ErrInsecureRedirect and ErrTooManyRedirects are *RedirectRefusedErrors;
• ErrTooManyRedirects behind a response-dropping middleware is not retried;
• a caller's policy that follows a redirect the generated policy would stop still follows it, and one that returns ErrUseLastResponse is kept.

Removing the wrap in SafeRedirects fails the new tests. go test ./..., go vet ./... and gofmt -l . are clean.

Breaking

A redirect refusal now arrives wrapped in a *RedirectRefusedError, and SafeRedirects always returns a copy of the client. Code that compares a refusal with == (such as urlErr.Err == ErrInsecureRedirect), checks what errors.Unwrap returns, uses a type switch on its own CheckRedirect error, or expects SafeRedirects(c) == c should use errors.Is or errors.As instead.