Fix binding 'C-Space' issue

We should check both 'ev.Key' and 'ev.Ch'. Because ev.Key is always '0'
if input does not have prefix('Ctrl-', 'Alt-') or input is not some
special key. If user binds some command to 'C-Space' or 'C-2', 'C-~'
(then that command is set to 'input.config.Keymap[0]'), the command
is executed by inputting non-prefix key like 'a', 'b', 'c'.
Merge pull request #72 from syohex/fix-key-zero-issue

Fix binding 'C-Space' issue