bugfix: avoid double `//` in links

When the base url contains a tailing `/` (e.g. your blog is hosted at the root of your page, then base is `/`), then the current replacement for non-relative links leads to creating double-`//` links, e.g.: `http://localhost:8080//articles/mine/image.jpeg`. This will lead the browser to look into `http://articles/mine/image.jpeg` instead of `http://localhost:8080/articles/mine/image.jpeg` (tested in Chrome and Firefox). Double-`//` also cause problems at other positions in the link (e.g. `http://localhost:8080/base//articles/mine/image.jpeg`; although some webservers may prevent this).