Unhighlighted code blocks contain invalid trailing newlines

[nicoburns]

This is the same issue as #278, except that it still occurs for codeblocks that are not syntax highlighted.

The easiest way to reproduce this is to comment out the line plugins.render.codefence_syntax_highlighter = Some(&adapter); in the syntect example. Or alternatively run the following code:

use comrak::{markdown_to_html, Options};

fn main() {
    let input = concat!("```Rust\n", "fn main<'a>();\n", "```");
    let formatted = markdown_to_html(input, &Options::default());
    println!("{}", formatted);
}

this prints

<pre><code class="language-Rust">fn main&lt;'a&gt;();
</code></pre>

rather than

<pre><code class="language-Rust">fn main&lt;'a&gt;();</code></pre>

[kivikakk]

Per #528 (comment), this behaviour is required by the spec; #278 was a bit different in that it had a leading newline as well.