Export serializable middleware and isPlain

Author: markerikson

src/index.js

@@ -2,6 +2,10 @@ export { configureStore, getDefaultMiddleware } from './configureStore'
 export { createReducer } from './createReducer'
 export { createAction, getType } from './createAction'
 export { createSlice } from './createSlice'
+export {
+  default as createSerializableStateInvariantMiddleware,
+  isPlain
+} from './serializableStateInvariantMiddleware'
 
 export { default as createNextState } from 'immer'
 export { combineReducers, compose } from 'redux'

src/serializableStateInvariantMiddleware.js

@@ -1,6 +1,6 @@
 import isPlainObject from './isPlainObject'
 
-function isPlain(val) {
+export function isPlain(val) {
   return (
     typeof val === 'undefined' ||
     typeof val === 'string' ||