title: rspec - matcher for one of choices
body: <p>I have method returning random value from the predefined array (namely: <code>[ 'value1', 'value2']</code>).
  How should I test that with rspec?</p>
  
  <p>I'd like to do something like:</p>
  
  <pre><code>expect(FooClass.new.value).to be_in ['value1', 'value2']
  </code></pre>
  
  <p>Any way to do that? Thanks.</p>