BUG-576: fixed bug in parsing of union types.

Parser creates an empty collection of restrictions and tries to set them to union builder, but union cannot be restricted.
This patch avoid adding restrictions to union builder.
Added test.

Change-Id: Ie0822f21380f8e5f9cd42960e7248927710053cd
Signed-off-by: Martin Vitez <mvitez@cisco.com>