Quote "on" to keep it from being loaded as bool

Fixes #39.

pillar.example

@@ -50,7 +50,7 @@ nginx:
         events:
           worker_connections: 768
         http:
-          sendfile: on
+          sendfile: 'on'
           include:
             - /etc/nginx/mime.types
             - /etc/nginx/conf.d/*.conf