title: Ruby koan 280 - where is the reference to to_str?
body: <p>I've had a look around and can't find this question:<br>
  For ruby koan 280 it's telling me the following underscore section should be false:  </p>
  
  <pre><code>  def test_to_str_allows_objects_to_be_treated_as_strings
      assert_equal __, File.exist?(CanBeTreatedAsString.new) # test passes, if __ is changed to false
    end
  </code></pre>
  
  <p>OK, fine. But how does this test that <code>to_str</code> allows objects to be treated as Strings? Here is the CanBeTreatedAsString class, which DOES include a <code>to_str</code> method:  </p>
  
  <pre><code>  class CanBeTreatedAsString
      def to_s
        "string-like"
      end
  
      def to_str
        to_s
      end
    end
  </code></pre>
  
  <p>...but how is that relevant to the assert_equal code above? Is it that <code>.exist?</code> expects a String? 
  This page:<br>
  <a href="http://www.ruby-doc.org/core-2.2.0/File.html#method-c-exist-3F" rel="nofollow">http://www.ruby-doc.org/core-2.2.0/File.html#method-c-exist-3F</a><br>
  says the parameter can be an IO object. Are some methods specific about the parameter types they receive? And if so, how do I tell?</p>