(Combined)LoggingHandler: Add Go 1.8 http.Pusher support

When building with Go 1.8, embed http.Pusher interface in
loggingResponseWriter interface and define a Push wrapper method on
responseLogger.  Without this change, users of handlers.LoggingHandler
or handlers.CombinedLoggingHandler cannot take advantage of the new
http.Pusher interface since responseLogger does not satisfy the
http.Pusher interface.

When building with pre-1.8 Go, the loggingResponseWriter and
responseLogger types remain unchanged.

handlers.go

@@ -94,7 +94,7 @@ func makeLogger(w http.ResponseWriter) loggingResponseWriter {
 	return logger
 }
 
-type loggingResponseWriter interface {
+type commonLoggingResponseWriter interface {
 	http.ResponseWriter
 	http.Flusher
 	Status() int

handlers_go18.go

@@ -0,0 +1,21 @@
+// +build go1.8
+
+package handlers
+
+import (
+	"fmt"
+	"net/http"
+)
+
+type loggingResponseWriter interface {
+	commonLoggingResponseWriter
+	http.Pusher
+}
+
+func (l *responseLogger) Push(target string, opts *http.PushOptions) error {
+	p, ok := l.w.(http.Pusher)
+	if !ok {
+		return fmt.Errorf("responseLogger does not implement http.Pusher")
+	}
+	return p.Push(target, opts)
+}

handlers_pre18.go

@@ -0,0 +1,7 @@
+// +build !go1.8
+
+package handlers
+
+type loggingResponseWriter interface {
+	commonLoggingResponseWriter
+}