The default behavior for Rails is to vary the response of an endpoint based on the `Accept:` header, and therefore it returns a `Vary:` header on responses. This instructs browsers and intermediate proxies to key their caches based on the value of the request's `Accept` header. In some cases (e.g. Akamai), the presence of a `Vary` header is enough to prevent caching entirely.
This commit restructures the Rails route definitions so that:
1. The "format" segment of the route is 'required'
2. The "format" segment of the route is constrained to a single value (e.g. `js` or `css`)
Now that the routes are guaranteed to have a `:format` segment, Rails will always prioritize that over the `Accept` header, and will therefore omit the `Vary` header.
Request specs are also added to test this behaviour for both stylesheets and theme-javascripts.
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