title: How to fix: Content-Type value is converted to lowercase
body: <p>I want to set content-type with lowercase and uppercase letters.</p>
  
  <p>I tried to set the content-type with <code>res.set('Content-Type', 'UPPERCASE-CONTENT-TYPE');</code> and also with <code>res.setHeader('Content-Type', 'UPPERCASE-CONTENT-TYPE');</code>. But the response I receive is then written in lower case: 'uppercase-content-type'.</p>
  
  <p>Is there a way to set the value without it being written in lower case?</p>