better handling for code blocks immediately following a paragraph

[yogthos]

Currently:

 "foo\n```\nbar\n```\nbaz"

renders as

"<p>foo<pre><code>bar\n</code></pre></p><p>baz</p>"

should render as

"<p>foo</p><pre><code>bar\n</code></pre><p>baz</p>"

[yogthos]

fixed by 7f93cec