title: How to clear the md-autocomplete cache?
body: <p>I'm using <code>md-autocomplete</code> to show results of an api query.  Attribute <code>md-items</code> is iterating over a promise: <code>item in getItems(searchText)</code>.</p>
  
  <p>This is working well, and using the cache subsequent uses of the same search text return immediately with the same results.</p>
  
  <p>But I need to be able to clear the cache at some points, when other search parameters change.  How can I do this?  By accessing the md-autocomplete controller perhaps?  Although that seems non-standard and I'm not sure how.</p>