Refactor RSS parser to use default namespace

This change avoid some limitations of the Go XML parser regarding XML namespaces

internal/reader/rss/atom.go

@@ -0,0 +1,43 @@
+// SPDX-FileCopyrightText: Copyright The Miniflux Authors. All rights reserved.
+// SPDX-License-Identifier: Apache-2.0
+
+package rss // import "miniflux.app/v2/internal/reader/rss"
+
+import "strings"
+
+type AtomAuthor struct {
+	Author AtomPerson `xml:"http://www.w3.org/2005/Atom author"`
+}
+
+func (a *AtomAuthor) String() string {
+	return a.Author.String()
+}
+
+type AtomPerson struct {
+	Name  string `xml:"name"`
+	Email string `xml:"email"`
+}
+
+func (a *AtomPerson) String() string {
+	var name string
+
+	switch {
+	case a.Name != "":
+		name = a.Name
+	case a.Email != "":
+		name = a.Email
+	}
+
+	return strings.TrimSpace(name)
+}
+
+type AtomLink struct {
+	URL    string `xml:"href,attr"`
+	Type   string `xml:"type,attr"`
+	Rel    string `xml:"rel,attr"`
+	Length string `xml:"length,attr"`
+}
+
+type AtomLinks struct {
+	Links []*AtomLink `xml:"http://www.w3.org/2005/Atom link"`
+}