title: Akka http Explicitly set HTTP header 'Content-Type: application/json' is ignored
body: <p>I have code like:</p>
  
  <pre><code>val request = HttpRequest(
        method = HttpMethods.POST,
        uri = Uri(url))
        .withHeaders(List(authorization, ctJson))
        .withEntity(ContentTypes.`application/json`, ByteString(jsonStr))
   Http().singleRequest(request)
  </code></pre>
  
  <p>which works fine but I have a warning during request:</p>
  
  <blockquote>
    <p>Explicitly set HTTP header 'Content-Type: application/json' is
    ignored, explicit <code>Content-Type</code> header is not allowed. Set
    <code>HttpRequest.entity.contentType</code> instead.</p>
  </blockquote>
  
  <p>How to resolve this issue ? Ive tried also:</p>
  
  <pre><code>val request = HttpRequest(
        HttpMethods.POST,
        uri = "https://api.sendgrid.com/v3/mail/send",
        entity = HttpEntity(ContentTypes.`application/json`, ByteString(jsonStr)),
        headers = List(authorization, ctJson))
  </code></pre>
  
  <p>but the same result..</p>