Make reducer generics more generic

[sbuzonas]

This enforces that the action parameter extends Action for strongly typed implementations, and defaults to AnyAction for the case where an action may not be explicitly typed.

Enhancement to #2467
Addresses an issue discovered in a failed build for DefinitelyTyped/DefinitelyTyped#17777

[sbuzonas]

This doesn't work because of the typescript version. It would need 2.3.

Should it be against next instead? The current latest version breaks a lot of types that extend Action rather than AnyAction

[sbuzonas]

Closing because I have no idea how to correctly fix this. Opened #2516 for discussion.