code: public String getNormalisedContentTypeValue() {
          String contentType = getHeader(CONTENT_TYPE);
          if (contentType != null) {
              return contentType.toLowerCase(Locale.ROOT);
          }
          return null;
      }
docs: Gets the normalised value of the (first) {@code Content-Type} header.
  <p>
  The normalisation is done by changing all characters to lower case.
  
  @return the value normalised, might be {@code null}.
  @since TODO add version
  @see #hasContentType(String...)