isObject should not be used by other functions in radash

[tiny-ant]

assign

since isObject(Object.create(null)) returns false, functions in radash like assign or keys do not match the real case in some business. It's a common situation that we should create a pure object without keys like toString to keep something accurate.
It's a better solution to provide another function util like isPlainObject.

[aleclarson]

Hi @tiny-ant, this issue is fixed in radashi@beta today (see #18). An official release is pending. We'd love to have you participate in the discussion and/or development of the radashi-org/radashi fork. 😄