Make sure there is always a space between <html and the block html_attribs

Hi! We wanted to add lang="en" attribute, so we put it in the block but we didn't bother with adding space. Like this:

    {% block html_attribs %}lang=en{% endblock %}

So the actual html tag became: <htmllang="en">. We didn't notice it for a while, so it seems like a minor issue, but it easy to fix