MD034 misses a bare URL whose scheme is not lower-case

`HTTP://www.example.com/` is a link on GitHub and MD034 says nothing about it. `mado` 0.3.2 reported it; #418 does not.

cmark-gfm decides the scheme with `strncasecmp`:

```c
if (link_len > len && strncasecmp((char *)link, valid_uris[i], len) == 0 &&
```

comrak 0.54 decides it with `==` (`src/parser/autolink.rs`):

```rust
let scheme = &subject.input[i - rewind..i];
let cond = |s: &&str| size - i + rewind >= s.len() && &scheme == s;
if !SCHEMES.iter().any(cond) {
return None;
}
```

so `Http://`, `HTTPS://` and the rest are text to comrak and links to GitHub. The GFM spec writes the schemes lower-case and says nothing about case either way, so cmark-gfm is the authority here and comrak is the one out of step.

MD034 reports the links its parser makes, which is what #418 settled, so there is nothing to fix in the rule: `rule::md034::tests::check_no_errors_with_upper_case_scheme` pins the loss and names this issue. What is left is to take it upstream to comrak and, once it lands, turn that test into one that expects a violation.

Not the same as `Www.example.com`, which cmark-gfm compares with `memcmp` and does not autolink either. mado agrees with cmark-gfm there; markdownlint reports it and is the one being lenient.