Allow strings to be used for selecting a single fix function for g:ale_fixers too

autoload/ale/fix.vim

@@ -225,7 +225,13 @@ function! ale#fix#Fix() abort
     let l:callback_list = []
 
     for l:sub_type in split(&filetype, '\.')
-        call extend(l:callback_list, get(g:ale_fixers, l:sub_type, []))
+        let l:sub_type_callacks = get(g:ale_fixers, l:sub_type, [])
+
+        if type(l:sub_type_callacks) == type('')
+            call add(l:callback_list, l:sub_type_callacks)
+        else
+            call extend(l:callback_list, l:sub_type_callacks)
+        endif
     endfor
 
     if empty(l:callback_list)

test/test_ale_fix.vader

@@ -107,3 +107,12 @@ Expect(Only the second function should be applied):
   $a
   $b
   $c
+
+Execute(ALEFix should handle strings for selecting a single function):
+  let g:ale_fixers.testft = 'AddCarets'
+  ALEFix
+
+Expect(The first function should be used):
+  ^a
+  ^b
+  ^c