Add section on unit testing middleware to docs

[idolize]

Updated WritingTests.md with a section on how to test middleware.

The function signature for middlware is a little confusing, so I think this could add a lot of value.

[gaearon]

Thanks

[gsklee]

Can we expand upon this part a little bit more? Like what's inside singleDispatch.js and an example actions.js showing how the singleDispatch middleware is being used in the code.

[tristanls]

What is singleDispatch.js? This example is extremely confusing. Is this fictional middleware? Am I supposed to know how to implement this module? The string singleDispatch only occurs in the doc file and nowhere else in this repo.

[markerikson]

Yeah, it appears to be a fictional middleware that is referenced in the unit test examples, but not actually shown anywhere.

[IndependentContractor]

Has anyone figured out how to implement singleDispatch or what it might look like?

[brigand]

@IndependentContractor my guess is that it'd either be one dispatch ever, or one dispatch per action type.

let dispatched = false;
const singleDispatch = store => next => action => {
  if (dispatched) return;
  dispatched = true;
  next(action);
}

let dispatchedTypes = new Map();
const singleDispatch = store => next => action => {
  if (dispatchedTypes.get(action.type)) return;
  dispatchedTypes.set(action.type, true);
  next(action);
}

[brigand]

I'm going to take a stab at improving these docs.

[brigand]

#2496