API: set content type for raw snippet

lib/api/projects.rb

@@ -141,6 +141,7 @@ module Gitlab
       #   GET /projects/:id/snippets/:snippet_id/raw
       get ":id/snippets/:snippet_id/raw" do
         @snippet = user_project.snippets.find(params[:snippet_id])
+        content_type 'text/plain'
         present @snippet.content
       end