remove initial assignment of dispatch in applyMiddleware to make it…

… more expressive

* separate local variables in applyMiddleware to make it more expressive

In the original version, the `dispatch` local variable in applyMiddleware stands for
different things before and after the `compose` call.
It'd be more expressive to separate "the original one on store" and "the
one with middlewares applied"

* refactor to make `applyMiddleware` clearer

pass `store.dispatch` to `compose`, not through another local variable

src/applyMiddleware.js

@@ -19,7 +19,7 @@ import compose from './compose'
 export default function applyMiddleware(...middlewares) {
  return (createStore) => (reducer, initialState, enhancer) => {
  var store = createStore(reducer, initialState, enhancer)
- var dispatch = store.dispatch
+ var dispatch
  var chain = []
 
  var middlewareAPI = {