test: header_with_url

[yutatokoi]

Same as PR #4, the commits in this PR were prioritised over:

fn code_block() {
        let input = "<pre><code class=\"language-rust\">let x: i32 = 123;</code></pre>".to_string();
        let expected = "```rust\nlet x: i32 = 123;```\n".to_string();
        assert_eq!(from_html_to_md(input), expected);
    }