title: Enzyme wrapper.state returns null
body: <p>I'm trying to check the default state of my component in my Enzyme unit tests. For this, I have the following component:</p>
  
  <pre><code>import React, { Component } from 'react';
  
  class Picker extends Component {
      constructor(props) {
          super(props);
          this.state = {
              sources: true
          };
      }
  }
  
  ...
  
  export default Picker;
  </code></pre>
  
  <p>Finally, my unit test looks like this:</p>
  
  <pre><code>it('should contain `everything` as a default value', () =&gt; {
    const wrapper = mount(&lt;Picker name='cnn' /&gt;);
    expect(wrapper.state('sources')).to.exist()
  });
  </code></pre>
  
  <p>The problem I'm facing here is that, I'm unable to get the component default state. The function wrapper.state('sources') should be returning 'true' if I'm not wrong.</p>
  
  <p>Is there any other step I'm missing? I'm using:</p>
  
  <ul>
  <li>"react": "^16.2.0"</li>
  <li>"chai": "^4.1.2",</li>
  <li>"enzyme": "^3.2.0",</li>
  <li>"enzyme-adapter-react-16": "^1.1.1",</li>
  <li>"jsdom": "^11.5.1",</li>
  <li>"mocha": "^4.0.1",</li>
  <li>"react-addons-test-utils": "^15.6.2"</li>
  </ul>