Fix for openpyxl < 2, and for issue pandas-dev#11408

If using openpyxl < 2, and value is a string that could be a number,
force a string to be written out.  If using openpyxl >= 2.2, then fix
issue pandas-dev#11408 to do with merging cells

Author: Dr-Irv

pandas/io/excel.py

@@ -708,7 +708,12 @@ def write_cells(self, cells, sheet_name=None, startrow=0, startcol=0):
         for cell in cells:
             colletter = get_column_letter(startcol + cell.col + 1)
             xcell = wks.cell("%s%s" % (colletter, startrow + cell.row + 1))
-            xcell.value = _conv_value(cell.val)
+            new_data_type = xcell.data_type_for_value(cell.val)
+            if (isinstance(cell.val, compat.string_types)
+                    and new_data_type != xcell.TYPE_STRING):
+                xcell.set_explicit_value(cell.val)
+            else:
+                xcell.value = _conv_value(cell.val)
             style = None
             if cell.style:
                 style = self._convert_to_style(cell.style)
@@ -1240,7 +1245,7 @@ def write_cells(self, cells, sheet_name=None, startrow=0, startcol=0):
                         start_row=startrow + cell.row + 1,
                         start_column=startcol + cell.col + 1,
                         end_column=startcol + cell.mergeend + 1,
-                        end_row=startrow + cell.mergeend + 1
+                        end_row=startrow + cell.mergestart + 1
                         )
 
                 # When cells are merged only the top-left cell is preserved

pandas/io/tests/test_excel.py

@@ -1116,8 +1116,6 @@ def test_to_excel_multiindex_cols(self):
                 fm = frame.columns.format(sparsify=False, 
                                           adjoin=False, names=False)                
                 frame.columns = [ ".".join(map(str, q)) for q in zip(*fm) ]
-                # Above not working on 2.7 and 3.5, so fake the columns
-                frame.columns = df.columns.copy()
             tm.assert_frame_equal(frame, df)
 
     def test_to_excel_multiindex_dates(self):