title: enzyme wrapper.instance() is null for connected component
body: <p>I am trying to test a connected react component, however once wrapping it, I cannot get the instance of it using instance(), it returns null. for non-connected components it does return the instance, what is the difference and how can i get an instance of a connected component?</p>
  
  <pre><code>it('connected component', () =&gt; {
    const wrapper = mount(
      &lt;Provider store={store}&gt;
        &lt;BrowserRouter&gt;
          &lt;ConnectedComponent /&gt;
        &lt;/BrowserRouter&gt;
      &lt;/Provider&gt;
    )
    const myComp = wrapper.find(ConnectedComponent)
    expect(myComp).toHaveLength(1) // passes
    console.log(myComp.instance()) //null
  
  })
  
  it('non-connected component', () =&gt; {
    const wrapper = mount(
      &lt;Provider store={store}&gt;
        &lt;BrowserRouter&gt;
          &lt;NonConnectedComponent /&gt;
        &lt;/BrowserRouter&gt;
      &lt;/Provider&gt;
    )
    const myComp = wrapper.find(NonConnectedComponent)
    expect(myComp).toHaveLength(1) // passes
    console.log(myComp.instance()) // prints the instancce
  })
  </code></pre>