fix: prevent body (and plaintextBody) from return html by accident

It is not clear why we ever would want to return the formatted_body when
we ask for the body, but it seems to not be used anywhere and there are
no tests covering that functionality. However it leads to suprising
results, where the plaintextBody can be tricked into returning html
without applying conversions. So we just get rid of that functionality.

lib/src/event.dart

@@ -292,7 +292,6 @@ class Event extends MatrixEvent {
   String get body {
     if (redacted) return 'Redacted';
     if (text != '') return text;
-    if (formattedText != '') return formattedText;
     return type;
   }
 

test/event_test.dart

@@ -1200,6 +1200,35 @@ void main() {
       }, room);
       expect(event.plaintextBody, '**blah**');
     });
+
+    test('body', () {
+      final event = Event.fromJson({
+        'type': EventTypes.Message,
+        'content': {
+          'body': 'blah',
+          'msgtype': 'm.text',
+          'format': 'org.matrix.custom.html',
+          'formatted_body': '<b>blub</b>',
+        },
+        'event_id': '\$source',
+        'sender': '@alice:example.org',
+      }, room);
+      expect(event.body, 'blah');
+
+      final event2 = Event.fromJson({
+        'type': EventTypes.Message,
+        'content': {
+          'body': '',
+          'msgtype': 'm.text',
+          'format': 'org.matrix.custom.html',
+          'formatted_body': '<b>blub</b>',
+        },
+        'event_id': '\$source',
+        'sender': '@alice:example.org',
+      }, room);
+      expect(event2.body, 'm.room.message');
+    });
+
     test('getDisplayEvent', () {
       final room = Room(id: '!1234', client: client);
       var event = Event.fromJson({