Implement http.ResponseWriter unwrapping like http.ResponseController

Since we rely on the connection not being hijacked too early (i.e.
detecting the presence of http.Hijacker) to set headers, we must
manually implement the unwrapping of the http.ResponseController. By
doing so, we also retain Go 1.19 compatibility without build tags.

Closes #455

accept.go

@@ -105,7 +105,7 @@ func accept(w http.ResponseWriter, r *http.Request, opts *AcceptOptions) (_ *Con
 		}
 	}
 
-	hj, ok := w.(http.Hijacker)
+	hj, ok := hijacker(w)
 	if !ok {
 		err = errors.New("http.ResponseWriter does not implement http.Hijacker")
 		http.Error(w, http.StatusText(http.StatusNotImplemented), http.StatusNotImplemented)

hijack.go

@@ -0,0 +1,33 @@
+//go:build !js
+
+package websocket
+
+import (
+	"net/http"
+)
+
+type rwUnwrapper interface {
+	Unwrap() http.ResponseWriter
+}
+
+// hijacker returns the Hijacker interface of the http.ResponseWriter.
+// It follows the Unwrap method of the http.ResponseWriter if available,
+// matching the behavior of http.ResponseController. If the Hijacker
+// interface is not found, it returns false.
+//
+// Since the http.ResponseController is not available in Go 1.19, and
+// does not support checking the presence of the Hijacker interface,
+// this function is used to provide a consistent way to check for the
+// Hijacker interface across Go versions.
+func hijacker(rw http.ResponseWriter) (http.Hijacker, bool) {
+	for {
+		switch t := rw.(type) {
+		case http.Hijacker:
+			return t, true
+		case rwUnwrapper:
+			rw = t.Unwrap()
+		default:
+			return nil, false
+		}
+	}
+}