title: Why tag lib test throws GrailsTagException?
body: <p>Let's say that I have one TagLib that use <code>FormatTagLib</code>:</p>
  
  <pre><code>class MyTagLib {
  
    def something = {attrs, body -&gt;
      def format = new FormatTagLib()
      out &lt;&lt; format.formatDate(attrs.date, format: 'HH:mm')
    }
  
  }
  </code></pre>
  
  <p>and I wrote a unit test for this taglib:</p>
  
  <pre><code>class MyTagLibTests extends TagLibUnitTestCase {
  
    //setUp() and tearDown() ommited
  
    void testMyTagLib() {
      tagLib = new MyTagLib()
      tagLib.something(date: Date.parse('20/04/2012 08:00','dd/MM/yyyy HH:mm'))
      assertEquals('08:00', out.toString()) //out is mocked...
    }
  
  }
  </code></pre>
  
  <p>Why this code throws exception for <code>formatDate</code>?</p>
  
  <pre><code>org.codehaus.groovy.grails.web.taglib.exceptions.GrailsTagException: Tag [formatDate] does not exist. No corresponding tag library found.
  </code></pre>