filer: processing all response headers, no pass through to volume server

* filer calculate MD5 etag
* filer handle response headers, instread of pass it to volume servers

weed/s3api/s3api_object_handlers.go

@@ -223,7 +223,6 @@ func (s3a *S3ApiServer) proxyToFiler(w http.ResponseWriter, r *http.Request, des
 
 	proxyReq.Header.Set("Host", s3a.option.Filer)
 	proxyReq.Header.Set("X-Forwarded-For", r.RemoteAddr)
-	proxyReq.Header.Set("Etag-MD5", "True")
 
 	for header, values := range r.Header {
 		for _, value := range values {