subscribe to throw when listener is not a function

[JamieDixon]

Fixes: #1324

[gaearon]

Let's check a few more data types: null, undefined, and string should throw.

[JamieDixon]

Added a few extra checks for the types mentioned. I've gone with the assertion style I've seen elsewhere rather than something like:

const cases = [undefined, null, ''];
    cases.map(arg =>
      expect(() =>
        store.subscribe(arg))
      .toThrow())

[gaearon]

Nit: please remove the extra newline

[JamieDixon]

+1 removed in latest commit.

[gaearon]

Out in 3.1.6, thanks!