`isPlainObject()` util returns is still incorrect

[alex89lj]

I am glad that this fix was merged(#1075), but it seems to be not perfect.
isPlainObject(Object.create(null))=>true,
isPlainObject(Object.create({}))=>false???.

This is correct in redux's "isPlainObject" function.
This is my PR:https://github.com/reduxjs/react-redux/pull/1005/files

[timdorr]

Please see #1074 for why we accepted that change and Mark's comments here for why we don't need to be exhaustive with our checks: #1005 (comment)