title: Content-type is blank in the headers of some requests
body: <p>I've ran this queries millions (yes, millions) of times before with other URLs. However, I'm getting a KeyError when checking the content-type of the following webpage.</p>
  
  <p>Code snippet:</p>
  
  <pre><code>r = requests.get("http://health.usnews.com/health-news/articles/2014/10/15/limiting-malpractice-claims-may-not-curb-costly-medical-tests", timeout=10, headers=headers)
  if "text/html" in r.headers["content-type"]:
  </code></pre>
  
  <p>Error: </p>
  
  <pre><code>KeyError: 'content-type'
  </code></pre>
  
  <p>I checked the content of <code>r.headers</code> and it's:</p>
  
  <pre><code>CaseInsensitiveDict({'date': 'Fri, 20 May 2016 06:44:19 GMT', 'content-length': '0', 'connection': 'keep-alive', 'server': 'BigIP'})
  </code></pre>
  
  <p>What could be causing this?</p>