Prevent superfluous response.WriteHeader (#15456) (#15476)

Backport #15456

This PR simply checks the status before writing the header.

Signed-off-by: Andrew Thornton <art27@cantab.net>

modules/context/response.go

@@ -4,7 +4,9 @@
 
 package context
 
-import "net/http"
+import (
+	"net/http"
+)
 
 // ResponseWriter represents a response writer for HTTP
 type ResponseWriter interface {
@@ -60,8 +62,10 @@ func (r *Response) WriteHeader(statusCode int) {
 		}
 		r.beforeExecuted = true
 	}
-	r.status = statusCode
-	r.ResponseWriter.WriteHeader(statusCode)
+	if r.status == 0 {
+		r.status = statusCode
+		r.ResponseWriter.WriteHeader(statusCode)
+	}
 }
 
 // Flush flush cached data