Async Ring handlers wrapper (#442)

* http/wrap-ring-async-handler helper, discussed in #439

* Fix typo in readme

Author: kachayev

README.md

@@ -8,7 +8,7 @@ Aleph exposes data from the network as a [Manifold](https://github.com/ztellman/
 
 ### HTTP
 
-Aleph follows the [Ring](https://github.com/ring-clojure) spec fully, and can be a drop-in replacement for any existing Ring-compliant server.  However, it also allows for the handler function to return a [Manifold deferred](https://github.com/ztellman/manifold) to represent an eventual response.  This feature may not play nicely with Ring middleware which modifies the response, but this can be easily fixed by reimplementing the middleware using Manifold's [let-flow](https://github.com/ztellman/manifold/blob/master/docs/deferred.md#let-flow) operator.
+Aleph follows the [Ring](https://github.com/ring-clojure) spec fully, and can be a drop-in replacement for any existing Ring-compliant server.  However, it also allows for the handler function to return a [Manifold deferred](https://github.com/ztellman/manifold) to represent an eventual response.  This feature may not play nicely with synchronous Ring middleware which modifies the response, but this can be easily fixed by reimplementing the middleware using Manifold's [let-flow](https://github.com/ztellman/manifold/blob/master/docs/deferred.md#let-flow) operator. `aleph.http/wrap-ring-async-handler` helper can be used to covert async 3-arity Ring handler to Aleph-compliant one.
 
 ```clj
 (require '[aleph.http :as http])

src/aleph/http.clj

@@ -388,3 +388,14 @@
    rather than a comma-delimited string."
   [^aleph.http.core.HeaderMap headers ^String k]
   (-> headers ^io.netty.handler.codec.http.HttpHeaders (.headers) (.getAll k)))
+
+(defn wrap-ring-async-handler
+  "Converts given asynchronous Ring handler to Aleph-compliant handler.
+
+   More information about asynchronous Ring handlers and middleware:
+   https://www.booleanknot.com/blog/2016/07/15/asynchronous-ring.html"
+  [handler]
+  (fn [request]
+    (let [response (d/deferred)]
+      (handler request #(d/success! response %) #(d/error! response %))
+      response)))