API HTTP::urlRewriter with (string)$code deprecated in 3.1. Fixed regressions and CSS urls.

urlRewriter will expect a callable as a second parameter,
but will work with the current api and simply raise a deprecation error.

HTTP::absoluteURLs now correctly rewrites urls into absolute urls. Resolves introduced in c56a80d6ce

HTTP::absoluteURLs now handles additional cases where urls were not translated.

Test cases for HTTP::absoluteURLs added for both css and attribute links.

Cleaned up replacement expression and improved documentation.