Bugfix in Control-W key binding. When there is only whitespace before the cursor, delete the whitespace.
Bugfix in Control-W key binding. When there is only whitespace before the cursor, delete the whitespace.

niklasf/python-prompt-toolkit,ALSchwalm/python-prompt-toolkit,jonathanslenders/python-prompt-toolkit,melund/python-prompt-toolkit,amjith/python-prompt-toolkit,ddalex/python-prompt-toolkit