Replace None with empty string before creating td

tablib/formats/_html.py

@@ -30,13 +30,17 @@ def export_set(dataset):
 	page.table.open()
 
 	if dataset.headers is not None:
+		new_header = [item if item is not None else '' for item in dataset.headers] 
+
 		page.thead.open()
-		headers = markup.oneliner.th(dataset.headers)
+		headers = markup.oneliner.th(new_header)
 		page.tr(headers)
 		page.thead.close()
 
 	for row in dataset:
-		html_row = markup.oneliner.td(row)
+		new_row = [item if item is not None else '' for item in row] 
+
+		html_row = markup.oneliner.td(new_row)
 		page.tr(html_row)
 
 	page.table.close()

test_tablib.py

@@ -224,6 +224,26 @@ class TablibTestCase(unittest.TestCase):
 
         self.assertEqual(html, self.founders.html)
 
+    def test_html_export_none_value(self):
+        """HTML export"""
+
+        html = markup.page()
+        html.table.open()
+        html.thead.open()
+
+        html.tr(markup.oneliner.th(['foo','', 'bar']))
+        html.thead.close()
+
+        html.tr(markup.oneliner.td(['foo','', 'bar']))
+
+        html.table.close()
+        html = str(html)
+
+        headers = ['foo', None, 'bar'];
+        d = tablib.Dataset(['foo', None, 'bar'], headers=headers)
+
+        self.assertEqual(html, d.html)
+
 
     def test_unicode_append(self):
         """Passes in a single unicode charecter and exports."""