Generate fields for all Identityrefs in an Union binding class

Generate fields for all Identityrefs in an Union binding class

Changed property name in generator for identityrefs so it will
generate variable names based on type of identityref and not
identityref_ like was before. With that it will generate fields
for all identityrefs that we have in union and methods for them.

Updated related test and added another one to cover the change.

JIRA: MDSAL-740
Change-Id: I11735655ab407f37774cb1b5ffa61955abaee37c
Signed-off-by: OleksandrZharov <Oleksandr.Zharov@pantheon.tech>
Signed-off-by: Tibor Král <tibor.kral@pantheon.tech>