Use 'set filetype' to set filetype once again (#467)

The s:setf() helper function was originally introduced to avoid setting
the filetype multiple times (375e9e1). I believe this covered the case
where a file had an .ex or .exs extension and also include "elixir" in
the shebang line (such that s:DetectElixir() would match and call 'set
filetype' redundantly).

c56bd82 removed the existing &filetype check in s:setf(), relying on the
&filetype global and ftdetect framework to avoid redundant updates.

I believe we can simplify this even further by using just 'set filetype'
in the file extension-based autocommands and including a guard against
(re)setting filetype in s:DetectElixir().

Also, use explicit case-insensitive comparison operators. This isn't a
good place to make assumptions about 'ignorecase'.

Author: jparise

ftdetect/elixir.vim

@@ -1,13 +1,9 @@
-au BufRead,BufNewFile *.ex,*.exs call s:setf('elixir')
-au BufRead,BufNewFile *.eex call s:setf('eelixir')
+au BufRead,BufNewFile *.ex,*.exs set filetype=elixir
+au BufRead,BufNewFile *.eex set filetype=eelixir
 au BufRead,BufNewFile * call s:DetectElixir()
 
-function! s:setf(filetype) abort
-  let &filetype = a:filetype
-endfunction
-
 function! s:DetectElixir()
-  if getline(1) =~ '^#!.*\<elixir\>'
-    call s:setf('elixir')
+  if &filetype !=# 'elixir' && getline(1) =~# '^#!.*\<elixir\>'
+    set filetype=elixir
   endif
 endfunction