Handle more encoding edge cases

- Feeds with charset specified only in Content-Type header and not in XML document
- Feeds with charset specified in both places
- Feeds with charset specified only in XML document and not in HTTP header

reader/encoding/encoding.go

@@ -0,0 +1,38 @@
+// Copyright 2018 Frédéric Guillot. All rights reserved.
+// Use of this source code is governed by the Apache 2.0
+// license that can be found in the LICENSE file.
+
+package encoding
+
+import (
+	"bytes"
+	"io"
+	"unicode/utf8"
+
+	"golang.org/x/net/html/charset"
+)
+
+// CharsetReader is used when the XML encoding is specified for the input document.
+//
+// The document is converted in UTF-8 only if a different encoding is specified
+// and the document is not already UTF-8.
+//
+// Several edge cases could exists:
+//
+// - Feeds with charset specified only in Content-Type header and not in XML document
+// - Feeds with charset specified in both places
+// - Feeds with charset specified only in XML document and not in HTTP header
+func CharsetReader(label string, input io.Reader) (io.Reader, error) {
+	var buf1, buf2 bytes.Buffer
+	w := io.MultiWriter(&buf1, &buf2)
+	io.Copy(w, input)
+	r := bytes.NewReader(buf2.Bytes())
+
+	if !utf8.Valid(buf1.Bytes()) {
+		// Transform document to UTF-8 from the specified XML encoding.
+		return charset.NewReaderLabel(label, r)
+	}
+
+	// The document is already UTF-8, do not do anything (avoid double-encoding)
+	return r, nil
+}