context: simplify stringify with a type switch

Minor style change.

Change-Id: Ib30243a71a83de1a67d3d005bfdd1e04265fca1e
GitHub-Last-Rev: 9d654de10eaa6f01ece29790fb81bc41dfd61eaf
GitHub-Pull-Request: golang/go#31479
Reviewed-on: https://go-review.googlesource.com/c/go/+/172199Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Sameer Ajmani <sameer@golang.org>

src/context/context.go

@@ -497,10 +497,10 @@ type valueCtx struct {
 // want context depending on the unicode tables. This is only used by
 // *valueCtx.String().
 func stringify(v interface{}) string {
-	if s, ok := v.(stringer); ok {
+	switch s := v.(type) {
+	case stringer:
 		return s.String()
-	}
-	if s, ok := v.(string); ok {
+	case string:
 		return s
 	}
 	return "<not Stringer>"