Improve compatibility with negroni and other http-based middleware.

[albrow]

Throttle looks like a really handy tool, and I'm interested in adapting it to work with negroni. This small change makes it easier to write a negroni-compatible middleware wrapper around the throttle handler.

For example, I can now write negroni-compatible middleware (which uses throttle behind the scenes) in just a few lines:

func Policy(quota *throttle.Quota, options ...*throttle.Options) negroni.HandlerFunc {
    return func(res http.ResponseWriter, req *http.Request, next http.HandlerFunc) {
        handler := throttle.Policy(quota, options...)
        handler(res, req)
        if res.Header().Get("X-Ratelimit-Remaining") == "0" {
            // the policy was triggered and the request should fail
            // in this case, we should not call next
        } else {
            // the policy was not triggered, and the request should continue
            // on to the next handler
            next(res, req)
        }
    }
}

I expect that this will also make throttle more compatible for those who prefer to roll their own middleware micro-frameworks. The tests still pass, and Policy still returns a handler which is directly compatible with martini, so as far as I know this change comes with no downsides.

[beatrichartz]

Nice work!

I don't see any downsides either, merging this.