http: add Flusher type; remove Flush from ResponseWriter

The Flush functionality wasn't removed, but now you have
to test if your ResponseWriter is also a Flusher:

func ServeHTTP(rw http.ResponseWriter, req *http.Request) {
   if f, ok := rw.(http.Flusher); ok {
       f.Flush()
   }
}

R=rsc, bradfitzwork
CC=gburd, golang-dev
https://golang.org/cl/4239077

src/pkg/http/server.go

@@ -70,7 +70,16 @@ type ResponseWriter interface {
 	// Thus explicit calls to WriteHeader are mainly used to
 	// send error codes.
 	WriteHeader(int)
+}
 
+// The Flusher interface is implemented by ResponseWriters that allow
+// an HTTP handler to flush buffered data to the client.
+//
+// Note that even for ResponseWriters that support Flush,
+// if the client is connected through an HTTP proxy,
+// the buffered data may not reach the client until the response
+// completes.
+type Flusher interface {
 	// Flush sends any buffered data to the client.
 	Flush()
 }