title: reactfire React+firebase "contains an invalid key (.key) in property"
body: <p>I get an error when trying to use update() or set()</p>
  
  <p>My relevant code:</p>
  
  <pre><code>const ref = new Firebase('https://whatever.firebase.org/employees');
  export default class EmployeeNew extends React.Component {
  ...
    this.update = () =&gt; {
      console.log(this.state, this.props.employee['.key']);
      ref.child(this.props.employee['.key']).set(this.state); // &lt;-- no-dice
      // ref.update(this.state); &lt;-- also fails.
    }
  ...
  }
  </code></pre>
  
  <p>Output of console logs, when update is invoked as written above:</p>
  
  <pre><code>Object {avatar: "https://somevalue.png", name: "Bananaman", .key: "-KDObp8r82Ornrrmfbk5"}
  Object "-KDObp8r82Ornrrmfbk5"
  </code></pre>
  
  <p>Error in browser console:</p>
  
  <pre><code>app.js:28366 Uncaught Error: Firebase.set failed: First argument  contains an invalid key (.key) in property 'employees.-KDObp8r82Ornrrmfbk5'.  Keys must be non-empty strings and can't contain ".", "#", "$", "/", "[", or "]"
  </code></pre>
  
  <p>Things i have tried:</p>
  
  <pre><code>  ref.update(this.state); // same error
    ref.update({this.props.employee['.key']: this.state}); // same error
  </code></pre>