title: Stubbing single exported function with Sinon
body: <p>I have just changed my lodash import from <code>import _ from 'lodash';</code> to <code>import debounce from 'lodash/debounce';</code><br>
  In my test I used to have <code>sandbox.stub(_, 'debounce').returnsArg(0);</code>, but now I'm stuck as to what to change it to. Obviously <code>sandbox.stub(debounce).returnsArg(0);</code> won't work. Not sure what to do when only a single function is exported from a module.</p>