Only add log.Lmicroseconds if log.Ltime is set

Also, move it out of Serve so it's possible to override.

goji.go

@@ -46,6 +46,9 @@ import (
 
 func init() {
 	bind.WithFlag()
+	if fl := log.Flags(); fl&log.Ltime != 0 {
+		log.SetFlags(fl | log.Lmicroseconds)
+	}
 }
 
 // Serve starts Goji using reasonable defaults.
@@ -54,8 +57,6 @@ func Serve() {
 		flag.Parse()
 	}
 
-	log.SetFlags(log.Flags() | log.Lmicroseconds)
-
 	// Install our handler at the root of the standard net/http default mux.
 	// This allows packages like expvar to continue working as expected.
 	http.Handle("/", DefaultMux)