Fix a PublicSuffix::DomainNotAllowed exception raised with viewing or editing a post
with a source like `Blog.`.
This happened when parsing the post's source. `Danbooru::URL.parse("Blog.")` would
heuristically parse the source into `http://blog`. Calling any methods related to the
URL's hostname or domain would lead to calling `PublicSuffix.parse("blog")`, which
would fail with PublicSuffix::DomainNotAllowed.