DOCS: show how to mock server responses in HTTP client tests

**Issue by [rkuhn](https://github.com/rkuhn)**
_Friday Jul 17, 2015 at 07:16 GMT_
_Originally opened as https://github.com/akka/akka/issues/18022_

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from the [mailing list](https://groups.google.com/d/topic/akka-user/-rsD467ADC4/discussion):

> I have an actor that is sending outbound http request.
> I would like top test that actor ,is there an option to mock that call ? what is the correct way for doing that ?

My answer in that thread:

> in order to switch out the actual HTTP part you’ll have to configure your Actor with this part, e.g. by providing a function “HttpRequest => Future[HttpResponse]” or a “Flow[HttpRequest, HttpResponse, _]” or similar. Then in your production code you plug in the HTTP client pool and in your tests you can simply stub it out by a function that provides canned answers.

Read the linked mailing-list discussion and locate the HTTP client testing documentation around actors, the HTTP client pool, and injectable request/response behavior. Done means the documentation answers how to replace outbound HTTP calls with canned responses when testing an actor.

Written by the indexing model from the issue text.