code: @Nonnull
    public final APIDescriptor addRequiredHeaders (@Nullable final String... aHeaderNames)
    {
      if (aHeaderNames != null)
        for (final String sHeaderName : aHeaderNames)
          addRequiredHeader (sHeaderName);
      return this;
    }
docs: Add one or more required HTTP headers. If one of these HTTP headers are not
  present, invocation will not be possible.
  
  @param aHeaderNames
  The names of the required HTTP headers. May be <code>null</code> or
  empty.
  @return this for chaining
  @see #addRequiredHeader(String)