Feat/reduce memory use for authorizer

[murasakiakari]

During the pervious content length issue, I also find that the authorizer will try to copy the whole data to buffer if the body is not a io.Seeker, which use extra memory when the body is bytes.Buffer.

In this pr, I try to shallow copy a bytes.Buffer for the retryBuf which does not need to copy entire buffer to memory but keep a separate copy for retry.