replaceReducer() should throw if argument is not a function

[gaearon]

This is an easy one, please feel free to send a PR.
We want to add a typeof check in replaceReducer just like we have when the store is created.

[Andersos]

Hello @gaearon, I tried making a pull request for this in #1320 . Do you want me to also try and make a test for the case?

[gaearon]

Thank you! Yes, tests are necessary.

[gaearon]

This is now fixed by #1321, thanks @dzhiriki!