Fix: stylesheet-html must have 'href' attribute

[attakei]

When using {% bootstrap_css %} if settings.py has 'theme_url', the link tag for theme_url is generated wrong.
Because sanitize_url_dict() always generate dict(src=url) from URL string.
But in tag, URL resource path must be in href attribute.

Except:

<link ref="stylesheet" href="/static/css/theme.css" >

Actually:

<link ref="stylesheet" src="/static/css/theme.css">

This PR fixes it.

[coveralls]

Coverage increased (+0.1%) to 91.182% when pulling c2c67d2 on attakei:fix-link-attrs into c444cea on zostera:develop.

[jieter]

thanks!