posts: fix exception when viewing post with source Blog..

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.

test/functional/posts_controller_test.rb

@@ -646,6 +646,15 @@ class PostsControllerTest < ActionDispatch::IntegrationTest
         end
       end
 
+      context "with a non-web source" do
+        should "render" do
+          @post.update!(source: "Blog.")
+          get post_path(@post)
+
+          assert_response :success
+        end
+      end
+
       should "respect the disable tagged filenames option in the Download link" do
         @user.update!(disable_tagged_filenames: true)
         get_auth post_path(@post), @user