title: .push end´s in "Do not mutate vuex store state outside mutation handlers"
body: <p>I have defined a Vuex store with (actions, state, mutations and getters)</p>
  
  <p>When i add a new todo to my state array in the mutations i got following error:
  Error: [vuex] Do not mutate vuex store state outside mutation handlers.</p>
  
  <pre><code>const state = {
    todos: []
  }
  
  export default {
    ADD_TODO (state, payload) {
      state.todos.push(payload.todo)
    }
  }
  </code></pre>
  
  <p>Is it not possible to use an array with the state?</p>
  
  <p>I also tried this with using an object for todos state which works fine.</p>