title: Unable to set correct BodyPart header in Gatling
body: <p>I have following scenario </p>
  
  <pre><code>exec(
          http("Request to endpoint: file")
            .post("http://localhost:8378/endpoint")
            .headers(Map(
              "Authorization" -&gt; "Bearer ${access_token}"
            ))
            .bodyPart(StringBodyPart("text_param", "text_value"))
            .bodyPart(RawFileBodyPart("file", "image.png")
              .contentType("application/octet-stream")
              .header("content-length", "35260")
            ).asMultipartForm
            .check(status.is(200))
        )
  </code></pre>
  
  <p>And, I get following headers for 'file' body part</p>
  
  <p><a href="https://i.stack.imgur.com/T2HhM.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/T2HhM.png" alt="enter image description here"></a></p>
  
  <p>Expected: content-length => 35260</p>
  
  <p>Actual: content-length35260</p>