Allow refine to change 'default' in leaf-list

RFC7950 allows 'default' to be specified for 'leaf-list's, which we
recognize. Unfortunately the definition of refine statement does not
take this into account. Fix that and add an explicit test.

JIRA: YANGTOOLS-1312
Change-Id: I0c3ba7ca7cabba486c7226d520ce82a7ec2d194b
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>
(cherry picked from commit 0125f41acf8764d48c8a78d71a34647631811853)