Add failing test for imported mixin with wrong path

When relative URLs are on, and there's an image referenced via url(...),
if the URL is unquoted, the generated CSS duplicates the base path
Merge branch 'unquoted-relative-url-in-mixin' of https://github.com/bdukes/less.js
Fix issue with urls in mixins
Fix issue with unqouted urls in mixins having their context evaluated twice. Fix sourcemap test output
changelog
Merge branch 'master' of https://github.com/cloudhead/less.js