Fix maktaba#test#Override to work reliably after vim 8.0.1378 (#212)
Fix maktaba#test#Override to work reliably after vim 8.0.1378 (#212)

Triggers vim's autoload mechanism instead of manually sourcing files in
the autoload function case, also fixing related bug in generated
autoload path so vim can find the autoload file.
google/vim-maktaba,malcolmr/vim-maktaba