Issue 442

This test currently passes **but it shouldn't**, so I've left it in the "closed issue" folder. 

The bug is due to a whitespace issue – libsass inserts whitespace when interpolating. https://github.com/sass/libsass/issues/442

Sass-spec ignores whitespace, so the test passes. I've filed a bug on the whitespace-ignoring issue here: https://github.com/sass/sass-spec/issues/69