Fix identity-ref value parsing/serialization

Fix identity-ref value parsing/serialization

When encountering a QName value in a leaf, a leaf-list or implied as the
value of a key, we need to properly encode it. The same is true when
parsing, where we have to consult current namespace mapping.

As a side-effect of this, we are also fixing the case of a leaf-list
entry referenced in a path argument -- which is important for all
non-String types.

JIRA: YANGTOOLS-1473
Change-Id: Id50ebd9a3c0c1378f1af8451b86c66e323757eba
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>
Signed-off-by: Ruslan Kashapov <ruslan.kashapov@pantheon.tech>