fix: retry on GOAWAY errors when using cached HTTP/2 connections

[0xuLiang]

RoundTripOnlyCachedConn bypasses the retry loop in RoundTripOpt, so GOAWAY and other retryable errors were returned directly to callers.

Now we check CanRetryError before returning, allowing retryable errors to fall through and create a new connection.

Changes:

• Export canRetryError as CanRetryError
• Check CanRetryError in roundTrip when RoundTripOnlyCachedConn fails

Fixes #376

[bertold]

I could not wait for a tagged release. If you need a release with the above fix you can get it from github.com/bertold/req v3.57.2 - note that this repo is a hard fork.

[0xuLiang]

I could not wait for a tagged release. If you need a release with the above fix you can get it from github.com/bertold/req v3.57.2 - note that this repo is a hard fork.

I checked in bertold/req and didn't see this fix included. I've submitted PR bertold#3 with the fix. Looking forward to your review.

[bertold]

I could not wait for a tagged release. If you need a release with the above fix you can get it from github.com/bertold/req v3.57.2 - note that this repo is a hard fork.

I checked in bertold/req and didn't see this fix included. I've submitted PR bertold#3 with the fix. Looking forward to your review.

Thanks for opening the PR. It has been merged. New version released. I also sent you an invite to be a collaborator on that repo.