Add DefaultUpstreamFlushInterval to replace magic time.Second value

pkg/upstream/http.go

@@ -6,7 +6,6 @@ import (
 	"net/http/httputil"
 	"net/url"
 	"strings"
-	"time"
 
 	"github.com/mbland/hmacauth"
 	"github.com/oauth2-proxy/oauth2-proxy/v7/pkg/apis/options"
@@ -100,7 +99,7 @@ func newReverseProxy(target *url.URL, upstream options.Upstream, errorHandler Pr
 	if upstream.FlushInterval != nil {
 		proxy.FlushInterval = upstream.FlushInterval.Duration()
 	} else {
-		proxy.FlushInterval = 1 * time.Second
+		proxy.FlushInterval = options.DefaultUpstreamFlushInterval
 	}
 
 	// InsecureSkipVerify is a configurable option we allow