whitespace: Skip '===' conflict marker for markdown

fixes: #2014

autoload/airline/extensions/whitespace.vim

@@ -52,7 +52,7 @@ endfunction
 function! s:conflict_marker()
   " Checks for git conflict markers
   let annotation = '\%([0-9A-Za-z_.:]\+\)\?'
-  if &ft is# 'rst'
+  if match(['rst', 'markdown'], &ft) >= 0
     " rst filetypes use '=======' as header
     let pattern = '^\%(\%(<\{7} '.annotation. '\)\|\%(>\{7\} '.annotation.'\)\)$'
   else