Specs and fixes for nested partials with relative paths problem

[kukushkin]

This fixes "File to import not found or unreadable" problem when there are several load paths, and imported partials use relative paths:
#27

I believe context.root_path is to blame:
https://github.com/petebrowne/sprockets-sass/blob/master/lib/sprockets/sass/importer.rb#L100
https://github.com/petebrowne/sprockets-sass/blob/master/lib/sprockets/sass/importer.rb#L63

There a two problems with that.

1. Once you start using several assets load paths, you essentially have several "roots" to build relative path from.

For example, a file A in assets/ imports a file B from vendor/. Now, if any @import directive in file B uses relative path, you would want to build this relative path from vendor/, but context.root_path points to assets/.

2. Sprockets::Context#resolve() treats relative paths starting with '.' or '..' as if they originated from context.pathname.dirname which is the actual directory where file A resides, not from context.root_path.
   https://github.com/sstephenson/sprockets/blob/master/lib/sprockets/context.rb#L78

Current sprockets-sass code builds relative paths from context.root_path, which in case of file A in assets/ and file B in vendor/ come in form ../vendor/. This works only when context.pathname.dirname and context.root_path are the same. I.e. if file A is at the topmost level in assets/. Placing it somewhere deeper, like in assets/folder/, breaks things.

My patch fixes these problems by finding appropriate root_path, so a relative path built from it is resolved properly by Sprockets::Context#resolve().

[petebrowne]

Thanks for this!

[kukushkin]

That was fast 8) thanks @petebrowne