Merging two AccessChanges should merge their visibility.

Previously a merge would ignore visibility changes entirely, now
it merges them as in apply.
Merge pull request #17 from Kobata/visibility-upgrade

Merging two AccessChanges should merge their visibility.
Don't overwrite existing AccessChange when adding duplicate.

This made the call to merge completely pointless.
Merge pull request #19 from Kobata/access-merging

Don't overwrite existing AccessChange when adding duplicate.