Eval endpoint cache control

[asdine]

To avoid having the result of the eval action being cached by proxies or browsers when evaluating the latest version of a ruleset, we need to add the right HTTP headers to control the caching policy:

• GET /rulesets/some-path?eval&param1=1&param2=2 -> latest version -> no http caching (or etag)
• GET /rulesets/some-path?eval&version=xyz&param1=1&param2=2 specific version -> http caching