title: How to include application/x-www-form-urlencoded HttpHeader in Akka-http 2.4.1?
body: <p>I am using Akka Http 2.4.1 to post a https request to the twitter api.</p>
  
  <p>According to their <a href="https://dev.twitter.com/oauth/application-only" rel="noreferrer">documentation</a>, I need two httpheaders.  Namely, Authorization and ContentType.</p>
  
  <p>To quote their docs:</p>
  
  <p><i>The request must include a Content-Type header with the value of application/x-www-form-urlencoded;charset=UTF-8.</i></p>
  
  <p>Here is my code:</p>
  
  <pre><code>val authorization = Authorization(BasicHttpCredentials(key, secret))
  
  /*
     This header is removed from the request with the following explanation!
     "Explicitly set HTTP header 'Content-Type: application/x-www-form-urlencoded;' is ignored, illegal RawHeader"
  */
  val contentType = RawHeader("Content-Type", "application/x-www-form-urlencoded;charset=UTF-8")
  
  Http().singleRequest(
      HttpRequest(
          uri = Uri("https://api.twitter.com/oauth2/token"),
          method = HttpMethods.POST,
          headers = List(authorization, contentType),
          entity = HttpEntity(`text/plain(UTF-8)`, "grant_type=client_credentials"),
          protocol = HttpProtocols.`HTTP/1.1`))
  </code></pre>
  
  <p>How can I include the <code>Content-Type</code> header with the value <code>application/x-www-form-urlencoded;charset=UTF-8</code> using Akka-http 2.4.1?</p>