code: @Nonnull
    public final APIDescriptor addRequiredHeader (@Nullable final String sHeaderName)
    {
      if (StringHelper.hasText (sHeaderName))
        m_aRequiredHeaders.add (sHeaderName);
      return this;
    }
docs: Add a required HTTP header. If this HTTP header is not present, invocation
  will not be possible.
  
  @param sHeaderName
  The name of the required HTTP header. May be <code>null</code> or
  empty in which case the header is ignored.
  @return this for chaining
  @see #addRequiredHeaders(String...)