title: http-outbound-gateway sets wrong content-type header
body: <p>we migrated from SIv3.x to SIv4.0.4 and found that now the http-outbound-gateway sets wrongly the content-type header. Instead of setting it like this content-type, it sets it like contentType. Searching around I found that this may be because now SI uses Spring Messaging clases where this attribute is named contentType.</p>
  
  <p>public static final String CONTENT_TYPE = "contentType";</p>
  
  <p>Is there a way to set correctly this content-type in the http-outbound or maybe before using an object-to-json-transformer?</p>
  
  <p>thanks in advance
  Regards
  Guzmán</p>