Add check for undefined middleware in configureStore

[Shrugsy]

Closes #937

As per commit message:

* Throw an error when a middleware creation callback
  is provided to configureStore that does not return
  an array of middleware

Two new tests are added:

• explicitly passing middleware: undefined should use default middleware (same as excluding the option)
• passing middleware: () => undefined should be considered a user error and throw an error

Note: While issue #937 arose due to a middleware creation function that returns undefined, I've considered "returning anything besides an array" as a user error in this context.

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[markerikson]

Nice. Two requests:

• We can make the check be only in dev mode, I think
• We might as well also verify that all the values in the array are functions and not undefined

[Shrugsy]

Nice. Two requests:

• We can make the check be only in dev mode, I think
• We might as well also verify that all the values in the array are functions and not undefined

I've added new commits to address these. For checking if each middleware in the array is actually a function, I've made it perform that check regardless of whether a middleware creation callback was used (i.e. passing a bad array to the middleware property directly will throw an error).

Question: Is there any kind of policy about squashing commits on pull requests, or will maintainers handle that?

[markerikson]

We've got "Squash and Merge" turned on for all the Redux repos. tbh I'm not exactly a fan, but eh :)