Honor Discard() in httpFancyWriter.ReadFrom

[DucMinhNe]

httpFancyWriter.ReadFrom only routes through basicWriter.Write when a tee writer is set. When it isn't, it streams straight to the original ResponseWriter's io.ReaderFrom — bypassing the discard flag entirely.

So after a caller invokes Discard() (to suppress the proxied output and write the response itself later), any body sent via ReadFrom/io.Copy — e.g. http.ServeContent, io.Copy(w, file), http.ServeFile — still reaches the client, defeating the purpose of Discard(). The plain Write path already respects discard; only the ReadFrom fast path missed it.

Fix

Extend the slow-path condition to also cover discard, so basicWriter.Write (which honors both tee and discard) handles those cases. This mirrors the existing tee handling in the same function:

if f.basicWriter.tee != nil || f.basicWriter.discard {
    n, err := io.Copy(&f.basicWriter, r)
    return n, err
}

The fast rf.ReadFrom path is now taken only for pure pass-through (no tee, no discard), where it's safe.

Test

Added TestHttpFancyWriterReadFromHonorsDiscard, which writes via ReadFrom after Discard() and asserts the original writer received nothing while BytesWritten() still reflects the consumed bytes. It fails on master (original receives all 11 bytes) and passes with the fix. Full go test ./middleware/ passes; gofmt/go vet clean.

[VojtechVitek]

LGTM. Thank you