Add ruby fixer using `rubocop --auto-correct` (#689)

* add ruby fixer for `rubocop --auto-correct`

autoload/ale/fix/registry.vim

@@ -42,6 +42,11 @@ let s:default_registry = {
 \       'suggested_filetypes': ['python'],
 \       'description': 'Fix Python files with yapf.',
 \   },
+\   'rubocop': {
+\       'function': 'ale#fixers#rubocop#Fix',
+\       'suggested_filetypes': ['ruby'],
+\       'description': 'Fix ruby files with rubocop --auto-correct.',
+\   },
 \}
 
 " Reset the function registry to the default entries.

autoload/ale/fixers/rubocop.vim

@@ -0,0 +1,31 @@
+" Set this option to change Rubocop options.
+if !exists('g:ale_ruby_rubocop_options')
+    " let g:ale_ruby_rubocop_options = '--lint'
+    let g:ale_ruby_rubocop_options = ''
+endif
+
+if !exists('g:ale_ruby_rubocop_executable')
+    let g:ale_ruby_rubocop_executable = 'rubocop'
+endif
+
+function! ale#fixers#rubocop#GetExecutable(buffer) abort
+    return ale#Var(a:buffer, 'ruby_rubocop_executable')
+endfunction
+
+function! ale#fixers#rubocop#GetCommand(buffer) abort
+    let l:executable = ale#Var(a:buffer, 'ruby_rubocop_executable')
+    let l:exec_args = l:executable =~? 'bundle$'
+    \   ? ' exec rubocop'
+    \   : ''
+
+    return ale#Escape(l:executable) . l:exec_args
+    \   . ' --auto-correct %t'
+
+endfunction
+
+function! ale#fixers#rubocop#Fix(buffer) abort
+    return {
+    \   'command': ale#fixers#rubocop#GetCommand(a:buffer),
+    \   'read_temporary_file': 1,
+    \}
+endfunction

test/fixers/test_rubocop_fixer_callback.vader

@@ -0,0 +1,28 @@
+Before:
+  Save g:ale_ruby_rubocop_executable
+
+  " Use an invalid global executable, so we don't match it.
+  let g:ale_ruby_rubocop_executable = 'xxxinvalid'
+
+  silent! execute 'cd /testplugin/test/command_callback'
+  silent cd ..
+  silent cd command_callback
+  let g:dir = getcwd()
+
+After:
+  Restore
+
+  silent execute 'cd ' . fnameescape(g:dir)
+  " Set the file to something else,
+  " or we'll cause issues when running other tests
+  silent file 'dummy.rb'
+  unlet! g:dir
+
+Execute(The rubocop callback should return the correct default values):
+  silent execute 'file ' . fnameescape(g:dir . '/ruby_paths/dummy.rb')
+
+  AssertEqual
+  \ {'read_temporary_file': 1,
+  \  'command': "'" . g:ale_ruby_rubocop_executable . "' "
+  \      . '--auto-correct %t' },
+  \ ale#fixers#rubocop#Fix(bufnr(''))