test: confirm that unknown lexers emit code blocks (#256)

Author: miketheman

tests/fixtures/test_rst_008.html

@@ -12,3 +12,4 @@
     <span class="nb">echo</span> <span class="s2">&quot;OK&quot;</span>
 <span class="k">fi</span></code></pre>
 <p>or click <a href="http://www.surveymonkey.com" rel="nofollow">SurveyMonkey</a></p>
+<pre><code>An unknown code fence block</code></pre>

tests/fixtures/test_rst_008.rst

@@ -20,3 +20,8 @@ and then here is some bash:
     fi
 
 or click `SurveyMonkey <http://www.surveymonkey.com>`_
+
+
+.. code-block:: abc
+
+    An unknown code fence block