fix Fn key sticks after typing arrow key

repro steps:
1. open Mac OS X’s Keyboard viewer.
2. run AXElement script below:
require ‘ax_elements’
app = AX::Application.new ‘com.apple.TextEdit’
set_focus_to app
type ‘\\DOWN’
type ‘(‘

After typing ‘(‘, from keyboard viewer, the ‘Fn’ key is still in
pressed state.

It looks like a Mac OS CGEventCreateKeyboardEvent’s bug. Currently the
workaround is to force unset ‘Fn’ flag after creating the event for
arrow keys.

There’re might be other keys having same issue.
Merge branch 'fix_arrow_keys' into my_master