Fix issues found in test.html

[cwalther]

These commits fix several issues that became apparent in the revival of test.html, #81.

1. Fix a copy-paste error that causes missing output because an exception aborted rendering when a certain error occurs. This makes the test.html output identical again to what is currently at http://lbovet.github.io/docson/tests/test.html (which I am treating as the “known good baseline” for the purposes of this consideration, even though it has some problems itself).
2. Fix aforementioned error: The HTML page was mistakenly retrieved in an attempt to get the root schema, resulting in a JSON parsing error.
3. Fix a test that started failing due to behavior changed by 2., however I consider that behavior change an improvement and therefore changed the test. This changes the same lines as #79, which is why the commits are based on that, not on master.
4. Fix backreferences to the root ({"$ref": "#"}) not appearing in the output. This is an improvement over the “baseline” test.html output.