allow default_classes to apply for ui.markdown

(fixes #1940)

nicegui/elements/markdown.py

@@ -22,7 +22,7 @@ class Markdown(ContentElement, component='markdown.js'):
         """
         self.extras = extras
         super().__init__(content=content)
-        self._classes = ['nicegui-markdown']
+        self._classes.append('nicegui-markdown')
         self._props['codehilite_css'] = (
             HtmlFormatter(nobackground=True).get_style_defs('.codehilite') +
             HtmlFormatter(nobackground=True, style='github-dark').get_style_defs('.body--dark .codehilite')