code: public final B contentType(CharSequence contentType) {
          requireNonNull(contentType, "contentType");
          autoDetectedContentType(false);
          return setHeader(HttpHeaderNames.CONTENT_TYPE, contentType);
      }
docs: Sets the {@code "content-type"} header. This method is a shortcut of:
  <pre>{@code
  builder.autoDetectedContentType(false);
  builder.setHeader(HttpHeaderNames.CONTENT_TYPE, contentType);
  }</pre>