code: @Nullable
    public String getHeader (@Nullable final String sName)
    {
      final ICommonsOrderedSet <String> aSet = m_aHeaders.get (_unifyHeaderName (sName));
      return aSet == null ? null : aSet.getFirst ();
    }
docs: Return the primary value for the given header, if any.
  <p>
  Will return the first value in case of multiple values.
  
  @param sName
  the name of the header
  @return the associated header value, or <code>null</code> if none