headerSorterPool.Put(sorter)
					return err
				}
			}
			if trace != nil && trace.WroteHeaderField != nil {
				formattedVals = append(formattedVals, v)
			}
		}
		if trace != nil && trace.WroteHeaderField != nil {
			trace.WroteHeaderField(kv.key, formattedVals)
			formattedVals = nil
		}
	}
	headerSorterPool.Put(sorter)
	return nil
}

	// failure.
	TLSHandshakeDone func(tls.ConnectionState, error)

	// WroteHeaderField is called after the Transport has written
	// each request header. At the time of this call the values
	// might be buffered and not yet written to the network.
	WroteHeaderField func(key string, value []string)

	// WroteHeaders is called after the Transport has written
	// all request headers.