Remove class="thead-default" from thead.

Remove class="thead-default" from thead.
This was preventing the class from being updated as part of the attrs as demonstrated in 
jieter/django-tables2#671

django_tables2/templates/django_tables2/bootstrap4.html

@@ -6,7 +6,7 @@
         <table {% render_attrs table.attrs class="table" %}>
             {% block table.thead %}
             {% if table.show_header %}
-                <thead class="thead-default" {{ table.attrs.thead.as_html }}>
+                <thead {{ table.attrs.thead.as_html }}>
                 <tr>
                 {% for column in table.columns %}
                     <th {{ column.attrs.th.as_html }}>