code: public void setContentType(MediaType mediaType) {
  		Assert.isTrue(!mediaType.isWildcardType(), "'Content-Type' cannot contain wildcard type '*'");
  		Assert.isTrue(!mediaType.isWildcardSubtype(), "'Content-Type' cannot contain wildcard subtype '*'");
  		set(CONTENT_TYPE, mediaType.toString());
  	}
docs: Set the {@linkplain MediaType media type} of the body,
  as specified by the {@code Content-Type} header.