title: New error when knitting get_engine(options$engine)
body: <p>I have an .Rmd file that previously had knitted without any problems.  Now I am getting the following error when executing this instruction</p>
  
  <pre><code>confusionMatrix(prediction1, ssTesting$classe)
  </code></pre>
  
  <p>The error is </p>
  
  <pre><code>Error in get_engine(options$engine) : Unknown language engine 'test' (must be registered via knit_engines$set())
  </code></pre>
  
  <p>If I execute each chunk independently the error does not occur.  </p>
  
  <p>It only happens if I <code>knit</code>.  </p>
  
  <p>The error is independent of using version R-3.1.2 or R-3.2.1.  I also set all the <code>cache options</code> to <code>FALSE</code> and made sure that all the libraries were correct but the error keeps appearing.</p>