HTML-escape all responses since JS and JSON responses can contain HTML

This reverts https://github.com/JangoSteve/remotipart/commit/7e350e8f311f32ba638a5db63862d59b23ec5309 to fix JS responses that contain unescaped HTML. Here is an example response that will fail without this fix:

```ruby
render js: 'console.log("uh oh &quot;")'
```