title: NSString string extension does not work
body: <p>I was trying out the GTMNSString+XML.h string extension for the following.</p>
  
  <pre><code>NSString *value = @"some text";
  NSString *newString = [value gtm_stringBySanitizingAndEscapingForXML];
  </code></pre>
  
  <p>However, this crashes with an exception:</p>
  
  <blockquote>
    <p>[__NSCFConstantString gtm_stringBySanitizingAndEscapingForXML]: unrecognized selector sent to instance 0x541f4</p>
  </blockquote>
  
  <p>GTMNSString+XML.h is defined <a href="http://code.google.com/p/google-toolbox-for-mac/source/browse/trunk/Foundation/GTMNSString%2BXML.m" rel="nofollow">here</a>.</p>
  
  <p>What could the problem be?</p>