Bug 762: Fixed accidental type change for union.

Previous patch used Java type derived from
subtypes from typedefinition, but most
of DOM code base were expecting string,
since in some cases it is hard to decide
between two subtypes.

Change-Id: I04355037047e742dc1e2a044dd7a5c48bc0dc0b7
Signed-off-by: Tony Tkacik <ttkacik@cisco.com>