code: @Nonnull
    public FineUploaderBasic setCustomHeaders (@Nullable final Map <String, String> aCustomHeaders)
    {
      m_aRequestCustomHeaders.setAll (aCustomHeaders);
      return this;
    }
docs: Additional headers sent along with the XHR POST request. Note that is option
  is only relevant to the ajax/XHR uploader.
  
  @param aCustomHeaders
  Custom headers to be set.
  @return this