Report.report_table: fix hheader shift when vheader present

Refs: #1560 (comment)

Author: kernc

Orange/canvas/report/report.py

@@ -257,6 +257,8 @@ def data(role=Qt.DisplayRole,
 
             if has_horizontal_header:
                 stream.append('<tr>')
+                if has_vertical_header:
+                    stream.append('<th></th>')
                 stream.extend(item_html(None, col) for col in columns)
                 stream.append('</tr>')