Custom wordcount formatters aren't autoloaded

[shayneholmes]

environment

• vim: 8.1.1050
• vim-airline: 86e73ca
• OS: MacOS 10.14.3
• Have you reproduced with a minimal vimrc: no
• What is your airline configuration: default
  if you are using terminal:
• terminal: iterm2
• $TERM variable: xterm256-color
• color configuration (:set t_Co?): 256
  if you are using Neovim:
  - does it happen in Vim: ???

setup

In .vimrc:

  let g:airline#extensions#wordcount#formatter = 'test'

~/.vim/autoload/airline/extensions/wordcount/formatters/test

function! airline#extensions#wordcount#formatters#test#to_string(wordcount)
  return "foo"
endfunction

actual behavior

Error detected while processing $HOME/.vim/autoload/airline/extensions/wordcount/formatters/test
line 3:
E746: Function name does not match script file name: airline#extensions#wordcount#formatters#test#to_string

expected behavior

Word count is replaced by "foo".

notes

The autoload error is fixed by moving the file to "test.vim", but then the file isn't executed at all, because airline is executing "autoload/airline/extensions/wordcount/formatters/test" instead of including the .vim extension.