title: Rspec / shoulda: testing, that a custom validator is called
body: <p>I have a custom validator (located in app/validators/uri_validator.rb) which is used in:</p>
  
  <pre><code>validates :link, uri: true
  </code></pre>
  
  <p>How do I specify this in my specs?</p>
  
  <p>Ideally I would like to have a one-line call, such as:</p>
  
  <pre><code>it { should validate_uri_of(:link) }
  </code></pre>
  
  <p>How do I do this?</p>