Fix conversion between unit and implement angles

This was harder than I expected, since I knew where to look for the conversion.
But as it turned out the conversion did never actually work correctly, due to a small bug.
Because of this I also [added some additional spec tests] [1] for the length units.
Fixes https://github.com/sass/libsass/issues/666

[1]: https://github.com/sass/sass-spec/pull/146