encoding/json: allow / and % in tag names

Fixes #2718

R=golang-dev, adg
CC=golang-dev
https://golang.org/cl/5532095

src/pkg/encoding/json/encode.go

@@ -419,8 +419,13 @@ func isValidTag(s string) bool {
 		return false
 	}
 	for _, c := range s {
-		if c != '$' && c != '-' && c != '_' && !unicode.IsLetter(c) && !unicode.IsDigit(c) {
-			return false
+		switch c {
+		case '$', '-', '_', '/', '%':
+			// Acceptable
+		default:
+			if !unicode.IsLetter(c) && !unicode.IsDigit(c) {
+				return false
+			}
 		}
 	}
 	return true

src/pkg/encoding/json/tagkey_test.go

@@ -36,6 +36,10 @@ type miscPlaneTag struct {
 	V string `json:"色は匂へど"`
 }
 
+type percentSlashTag struct {
+	V string `json:"text/html%"` // http://golang.org/issue/2718
+}
+
 type emptyTag struct {
 	W string
 }
@@ -68,6 +72,7 @@ var structTagObjectKeyTests = []struct {
 	{misnamedTag{"Animal Kingdom"}, "Animal Kingdom", "X"},
 	{badFormatTag{"Orfevre"}, "Orfevre", "Y"},
 	{badCodeTag{"Reliable Man"}, "Reliable Man", "Z"},
+	{percentSlashTag{"brut"}, "brut", "text/html%"},
 }
 
 func TestStructTagObjectKey(t *testing.T) {
@@ -88,7 +93,7 @@ func TestStructTagObjectKey(t *testing.T) {
 					t.Fatalf("Unexpected value: %#q, want %v", s, tt.value)
 				}
 			default:
-				t.Fatalf("Unexpected key: %#q", i)
+				t.Fatalf("Unexpected key: %#q, from %#q", i, b)
 			}
 		}
 	}