add support for async RSAA properties

[bareynol]

When defining an RSAA action, you can define the properties of the RSAA action (body, headers, options) to be either a plain JavaScript object or a function.

If a function, the middleware executes the function to attain the result. The middleware that is executing the function is already an async function, so I added an await before the call of the above RSAA properties, so that async functions could be passed in and still evaluated.

To give a real-world example of why this is necessary: I store credentials for an API on the keychain in React-Native. I set the headers property to be a function that reads the keychain and formats the Authorization header. Accessing the keychain is done asynchronously. This change allows an async function to be used, and does not affect synchronous functions.

[coveralls]

Coverage remained the same at 100.0% when pulling 775366b on echolocation:master into 6e782cd on agraboso:master.

[jamesjara]

looks good

[nason]

Thank you for the contribution @echolocation! This looks great.

Would you mind adding at least one test case covering this behavior? Something like https://github.com/agraboso/redux-api-middleware/blob/master/src/middleware.test.js#L431 but with an async function?

[bareynol]

@nason certainly! Thank you for pointing me directly at a relevant test case.

I've added async versions of the tests for the properties: headers, endpoint, body, and options.

It's the same test for each, but it uses mockResolvedValue to mock async functions

[nason]

Awesome, thanks for adding tests! This looks great. I'll publish a new release with these changes shortly.