Fix zoom mode when drag_button set to None.
Fix zoom mode when drag_button set to None.

When user set `drag_button=None` on the `ZoomTool` and enters
zoom mode (by pressing 'z'), the left-button would start zoom-box,
but release the left-button did not end selection.

burnpanck/chaco,tommy-u/chaco,tommy-u/chaco,burnpanck/chaco,burnpanck/chaco,tommy-u/chaco