encoding/xml: case-insensitive encoding recognition

From the XML spec: "XML processors should match character encoding
names in a case-insensitive way"

Fixes #12417.

Change-Id: I678c50152a49c14364be62b3f21ab9b9b009b24b
Reviewed-on: https://go-review.googlesource.com/14084Reviewed-by: Russ Cox <rsc@golang.org>

src/encoding/xml/xml.go

@@ -583,7 +583,7 @@ func (d *Decoder) rawToken() (Token, error) {
 				return nil, d.err
 			}
 			enc := procInst("encoding", content)
-			if enc != "" && enc != "utf-8" && enc != "UTF-8" {
+			if enc != "" && enc != "utf-8" && enc != "UTF-8" && !strings.EqualFold(enc, "utf-8") {
 				if d.CharsetReader == nil {
 					d.err = fmt.Errorf("xml: encoding %q declared but Decoder.CharsetReader is nil", enc)
 					return nil, d.err

src/encoding/xml/xml_test.go

@@ -771,3 +771,35 @@ func TestIssue11405(t *testing.T) {
 		}
 	}
 }
+
+func TestIssue12417(t *testing.T) {
+	testCases := []struct {
+		s  string
+		ok bool
+	}{
+		{`<?xml encoding="UtF-8" version="1.0"?><root/>`, true},
+		{`<?xml encoding="UTF-8" version="1.0"?><root/>`, true},
+		{`<?xml encoding="utf-8" version="1.0"?><root/>`, true},
+		{`<?xml encoding="uuu-9" version="1.0"?><root/>`, false},
+	}
+	for _, tc := range testCases {
+		d := NewDecoder(strings.NewReader(tc.s))
+		var err error
+		for {
+			_, err = d.Token()
+			if err != nil {
+				if err == io.EOF {
+					err = nil
+				}
+				break
+			}
+		}
+		if err != nil && tc.ok {
+			t.Errorf("%q: Encoding charset: expected no error, got %s", tc.s, err)
+			continue
+		}
+		if err == nil && !tc.ok {
+			t.Errorf("%q: Encoding charset: expected error, got nil", tc.s)
+		}
+	}
+}