Fix css-file read in windows

url.resolve does not resolve correct url in windows since it doesn't
like backslashes.

If we inline following file:
````
<html>
<head>
    <link rel="stylesheet" href="css/style.css">
</head>
<body>
</body>
````

getHrefContent returns undefined in Windows, although in Linux the css-file is read just fine. This does not effect other operating systems.