[#6515][WCF]: Enable custom headers for NET_2_1.

[#6515][WCF]: Enable custom headers for NET_2_1.

This allows

	HttpRequestMessageProperty request = new HttpRequestMessageProperty ();
	request.Headers.Add ("Foo", "Bar");

	OperationContext.Current.OutgoingMessageProperties.Add (HttpRequestMessageProperty.Name, request);

The code was already there, but disabled for the NET_2_1 profile.