URL containing filename doesn't resolve relative path correctly

[dezull]

For example, from http://example.com/file.html:

relative path foo resolves to http://example.com/file.html/foo

but the correct behavior should be:

http://example.com/foo

I have written a test and possible fix here https://github.com/dezull/rubyretriever/commit/45114e313c9d73e43ce13cf61c687f65dedb18df