fix(xep): Fix parsing user avatar data with newlines

packages/moxxmpp/lib/src/xeps/xep_0084.dart

@@ -16,6 +16,7 @@ abstract class AvatarError {}
 
 class UnknownAvatarError extends AvatarError {}
 
+/// The result of a successful query of a users avatar.
 class UserAvatarData {
   const UserAvatarData(this.base64, this.hash);
 
@@ -26,7 +27,9 @@ class UserAvatarData {
   final String hash;
 
   /// The raw avatar data.
-  List<int> get data => base64Decode(base64);
+  /// NOTE: Remove newlines because "Line feeds SHOULD NOT be added but MUST be accepted"
+  /// (https://xmpp.org/extensions/xep-0084.html#proto-data).
+  List<int> get data => base64Decode(base64.replaceAll('\n', ''));
 }
 
 class UserAvatarMetadata {

packages/moxxmpp/test/xeps/xep_0084_test.dart

@@ -0,0 +1,14 @@
+import 'dart:convert';
+import 'package:moxxmpp/moxxmpp.dart';
+import 'package:test/test.dart';
+
+void main() {
+  test('Test accepting newlines', () {
+    const data = UserAvatarData(
+      'cGFwYXR1d\nHV3\n\nYXdh',
+      'some-id',
+    );
+
+    expect(utf8.decode(data.data), 'papatutuwawa');
+  });
+}