write_textfield with clear=True cause problem with MAC OS

[adam]

In actions.py, on line 556, key combination of 'control+a' is used to select all letters in the field and use delete key to empty it. But this cannot work on MAC OS since MAC OS use 'cmd+a' as the select all combination. And since it requires 'cmd+a', 'control+1' on MAC OS will generate an unwanted 'a' which cause problem. Furthermore, I tried to use "textfield.send_keys(keys.Keys().COMMAND, 'a')" with Safari on MAC OS. But it seemed this is not working, you also got an unwanted 'a' in front.