result: fix nsfw->flags rename, fix flag merging, and add tests for flag reporting/merging

The rename from nsfw->flags in linkcache.py didn't get mirrored in
result.py, so flags were getting ignored.

The ~NSFW->NSFW transition was incorrectly implemented. I've added tests
to ensure correctness there.

Signed-off-by: Jeff Mahoney <jeffm@jeffreymahoney.com>
linkcache: catch AssertionError separately

The except Exception catch-all is wrong and needs to be finer grained. This
will happen gradually, but we can start by catching AssertionError.

Signed-off-by: Jeff Mahoney <jeffm@jeffreymahoney.com>
Merge pull request #3 from jeffmahoney/to-push

To push