title: ExpectedException.expectMessage((String) null) is not working
body: <p>I am writing JUnit4 unit tests and have a condition where I need to assert that an exception was thrown with <code>null</code> message.</p>
  
  <pre><code>@Rule
  public final ExpectedException exception = ExpectedException.none();
  
  @Test
  public final void testNullException() throws Exception {
      exception.expect(Exception.class);
      exception.expectMessage((String) null);
      mPackage.getInfo(null);
  }
  </code></pre>
  
  <p>The line <code>mPackage.getInfo(null)</code> is throwing an Exception with <code>null</code> message properly, but the JUnit test is failing with the message:</p>
  
  <pre><code>java.lang.AssertionError: 
  Expected: (an instance of java.lang.Exception and exception with message a string containing null)
       but: exception with message a string containing null message was null
  </code></pre>
  
  <p>Is there anyway to test for an exception with a <code>null</code> message in <strong>JUnit4 way</strong>. (I know I can catch the exception and check for the conditions myself).</p>