add GetBody in hijack LoadResponse request for http2 retry

[pyneda]

Fix for the following error

(http2: Transport: cannot retry err [stream error: stream ID 1; PROTOCOL_ERROR; received from peer] after Request.Body was written; define Request.GetBody to avoid this error)

[ysmood]

Could you add at least a test for it?

[pyneda]

Could you add at least a test for it?

Attempted to set up a test case using an HTTP/2 test server and a client that forces retries to reproduce the original issue. However, I wasn't able to replicate the same error within a test case.

As a fallback, added a simpler test that ensures the request body is correctly propagated through redirects and verifies that the GetBody function added in LoadResponse behaves as expected.

Some related issues:
stripe/stripe-go#710
connectrpc/connect-go#541