title: Imported styles object is empty in Jest
body: <p>I have a component:</p>
  
  <pre><code>import React from 'react';
  import * as styles from './RedComponent.css';
  
  class RedComponent extends React.Component {
    render () {
      return &lt;div className={ styles.red }&gt;&lt;/div&gt;;
    }
  }
  </code></pre>
  
  <p>This is the test case:</p>
  
  <pre><code>describe('Test suite', () =&gt; {
    test('RedComponent tests', () =&gt; {
      const wrapper = shallow(&lt;RedComponent /&gt;);
  });
  </code></pre>
  
  <p><code>console.log(wrapper.debug());</code> 
  gives </p>
  
  <pre><code>&lt;div className={[undefined]}&gt;&lt;/div&gt; 
  </code></pre>
  
  <p>instead of </p>
  
  <pre><code>&lt;div className="RedComponent__red"&gt;&lt;/div&gt;
  </code></pre>
  
  <p>If I console the imported styles I get </p>
  
  <pre><code>console.log(styles); // {default: {}}
  </code></pre>
  
  <p>This is only in Jest test cases. Style is not undefined when the app renders in browser.</p>
  
  <p>My jest config:</p>
  
  <pre><code>{
    "moduleFileExtensions": [
      "js"
    ],
    "moduleDirectories": [
      "node_modules"
    ],
    "moduleNameMapper": {
      "\\.(css|less)$": "identity-obj-proxy"
    },
    "setupFiles": [
      "./test-setup.js"
    ],
    "collectCoverageFrom": [
      "src/**/*.{js}",
      "!**/node_modules/**"
    ],
    "testEnvironment": "node",
    "transform": {
      "^.+\\.js$": "babel-jest",
      "\\.(md|ttf|txt|eot|ico|otf|svg|png|gif|woff2|woff|jpeg)$": "./file-transformer.js"
    }
  }
  </code></pre>
  
  <p>Using jest v21.2.1, identity-obj-proxy v3.0.0 and React v16.0.0.</p>