Added more Http Headers & fixed ETag header

With this PR `play.mvc.Http.HeaderNames` is now up-to-date with [play.api.http.HeaderNames](https://github.com/playframework/playframework/blob/master/framework/src/play/src/main/scala/play/api/http/StandardValues.scala#L215)

Plus: It's `ETag` not `Etag` - see http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.19