Ensure response_body is escaped in html_example template

[cam-woloshyn]

This commit resolves an issue for HTML documents when the response_body includes HTML markup.

For example, if response_body = {"email":"email1@example.com","name":"<h1>Order 1</h1>","paid":true}
the generated HTML page won't escape the HTML, and <h1>Order 1</h1> will be rendered as a Heading 1. This is so because the mustache template renders the response_body using triple curly braces rather than double curly braces. Triple curly braces were introduced to pretty print the JSON. But this functionality was later removed, and I can't see why triple braces are still needed.

(Note: this issue only affects Rails 3 but not Rails 4 apps. I'm not certain why, but I believe it has to do with the changes to JSON handling (see release notes). This patch should be a transparent change for Rails 4 apps.)

[oestrich]

Thanks, especially for researching what broke!