refactor(reducers): use handleActions for dropdown-reducer

Removes a comment that is no longer true.
Also replaces some Symbol keys with strings so that state is serializable.

server/zanata-frontend/src/frontend/app/editor/reducers/dropdown-reducer.js

@@ -1,44 +1,26 @@
-import updateObject from 'immutability-helper'
+import update from 'immutability-helper'
+import { handleActions } from 'redux-actions'
 import {
   OPEN_DROPDOWN, CLOSE_DROPDOWN, TOGGLE_DROPDOWN
 } from '../actions/action-types'
 
 const defaultState = {
   openDropdownKey: undefined,
-  docsKey: Symbol('Documents Dropdown'),
-  localeKey: Symbol('Locales Dropdown'),
-  uiLocaleKey: Symbol('UI Locales Dropdown')
+  docsKey: 'Documents Dropdown',
+  localeKey: 'Locales Dropdown',
+  uiLocaleKey: 'UI Locales Dropdown'
 }
 
-/**
- * State is just the key of the currently open dropdown, or undefined if
- * no dropdown is open.
- */
-const dropdownReducer = (state = defaultState, action) => {
-  switch (action.type) {
-    case OPEN_DROPDOWN:
-      return update({
-        openDropdownKey: {$set: action.payload}
-      })
+const dropdownReducer = handleActions({
+  [OPEN_DROPDOWN]: (state, { payload }) =>
+    update(state, { openDropdownKey: {$set: payload} }),
 
-    case CLOSE_DROPDOWN:
-      return update({
-        openDropdownKey: {$set: undefined}
-      })
+  [CLOSE_DROPDOWN]: state =>
+    update(state, { openDropdownKey: {$set: undefined} }),
 
-    case TOGGLE_DROPDOWN:
-      const isOpen = action.payload === state.openDropdownKey
-      return update({
-        openDropdownKey: {$set: isOpen ? undefined : action.payload}
-      })
-
-    default:
-      return state
-  }
-
-  function update (commands) {
-    return updateObject(state, commands)
-  }
-}
+  [TOGGLE_DROPDOWN]: (state, { payload }) => update(state, { openDropdownKey:
+    {$set: payload === state.openDropdownKey ? undefined : payload}
+  })
+}, defaultState)
 
 export default dropdownReducer