title: how to syncState to firebase with re-base?
body: <p>I'm trying to persist data with re-base but with this setup, I'm getting nothing. When I try to <code>addExpense</code> I'm getting no errors and no new object.</p>
  
  <h2>My re-base setup here:</h2>
  
  <pre><code>class ExpensesContainer extends Component {
    constructor(props) {
      super(props);
      this.state = {
        expenses: {
          expenseList: []
        }
      }
    }
  
    componentDidMount(){
      base.syncState(`/`, {
        context: this,
        state: 'expenses.expenseList',
        asArray: true
      });
    }
  
    addExpense = (expense) =&gt; {
      this.setState({
        expenses: this.state.expenses.expenseList.concat([expense]) //updates Firebase and the local state
      });
    }
  </code></pre>
  
  <p>If I'm not using componentDidMount then my addExpense action looks like this and everything works well:</p>
  
  <pre><code>  // addExpense = (expense) =&gt; {
    //   const expenseList = [...this.state.expenses.expenseList, expense];
    //   this.setState({expenses: {expenseList}});
    // }
  </code></pre>