Add test for passing None to add_formatter

src/tablib/core.py

@@ -643,7 +643,7 @@ def add_formatter(self, col, handler):
             else:
                 raise KeyError
 
-        if not col > self.width:
+        if col is None or col <= self.width:
             self._formatters.append((col, handler))
         else:
             raise InvalidDatasetIndex

tests/test_tablib.py

@@ -578,6 +578,22 @@ def _formatter(cell_value):
         # Test once more as the result should be the same
         self.assertEqual(self.founders.dict, expected)
 
+    def test_formatters_all_cols(self):
+        """
+        Passing None as first add_formatter param apply formatter to all columns.
+        """
+
+        def _formatter(cell_value):
+            return str(cell_value).upper()
+
+        self.founders.add_formatter(None, _formatter)
+
+        self.assertEqual(self.founders.dict, [
+            {'first_name': 'JOHN', 'last_name': 'ADAMS', 'gpa': '90'},
+            {'first_name': 'GEORGE', 'last_name': 'WASHINGTON', 'gpa': '67'},
+            {'first_name': 'THOMAS', 'last_name': 'JEFFERSON', 'gpa': '50'},
+        ])
+
     def test_unicode_renders_markdown_table(self):
         # add another entry to test right field width for
         # integer