ShellDetect: fall back to filetype if no hashbang

Some files lack a hashbang line but still have an unambiguous filetype.
For example, the file `.zshrc` has the filetype `zsh`.

Augment ale#handlers#sh#GetShellType to fall back to the filetype if
no hashbang line can be found.

autoload/ale/handlers/sh.vim

@@ -4,17 +4,24 @@
 function! ale#handlers#sh#GetShellType(buffer) abort
     let l:bang_line = get(getbufline(a:buffer, 1), 0, '')
 
+    let l:command = ''
+
     " Take the shell executable from the hashbang, if we can.
     if l:bang_line[:1] is# '#!'
         " Remove options like -e, etc.
         let l:command = substitute(l:bang_line, ' --\?[a-zA-Z0-9]\+', '', 'g')
-
-        for l:possible_shell in ['bash', 'dash', 'ash', 'tcsh', 'csh', 'zsh', 'ksh', 'sh']
-            if l:command =~# l:possible_shell . '\s*$'
-                return l:possible_shell
-            endif
-        endfor
     endif
 
+    " If we couldn't find a hashbang, try the filetype
+    if l:command is# ''
+        let l:command = &filetype
+    endif
+
+    for l:possible_shell in ['bash', 'dash', 'ash', 'tcsh', 'csh', 'zsh', 'ksh', 'sh']
+        if l:command =~# l:possible_shell . '\s*$'
+            return l:possible_shell
+        endif
+    endfor
+
     return ''
 endfunction

test/test_shell_detection.vader

@@ -98,6 +98,16 @@ Execute(The ksh dialect should be used for shellcheck if b:is_kornshell is 1):
 
   AssertEqual 'ksh', ale_linters#sh#shellcheck#GetDialectArgument(bufnr(''))
 
+Execute(The filetype should be used as the default shell type when there is no hashbang line):
+  set filetype=zsh
+  AssertEqual 'zsh', ale#handlers#sh#GetShellType(bufnr(''))
+
+  set filetype=tcsh
+  AssertEqual 'tcsh', ale#handlers#sh#GetShellType(bufnr(''))
+
+  set filetype=python
+  AssertEqual '', ale#handlers#sh#GetShellType(bufnr(''))
+
 Given(A file with /bin/ash):
   #!/bin/ash