code: @Nonnull
    public final UnifiedResponse setETagIfApplicable (@Nonnull @Nonempty final String sETag)
    {
      if (m_eHttpVersion.isAtLeast11 ())
        setETag (sETag);
      return this;
    }
docs: Set an ETag for the response if this is an HTTP/1.1 response. HTTP/1.0 does
  not support ETags. The ETag must be a quoted value (being surrounded by
  double quotes).
  
  @param sETag
  The quoted ETag to be set. May neither be <code>null</code> nor
  empty.
  @return this