Fixed @extend in mixin. Closes #638
Fixed @extend in mixin. Closes #638

some cases still fail

Ricardh522/stylus,jameswomack/stylus,HossainKhademian/Stylus,a-x-/stylus,jonpacker/stylus,SimenB/stylus,modulexcite/stylus,Delapouite/stylus,trixter5/stylus,stylus/stylus,behzad88/stylus,darwin/stylus,meteor/stylus,Panya/stylus,baotingfang/stylus,tyv/stylus,yetone/stylus,ThiagoGarciaAlves/stylus,galenvagen/stylus,angryrobot/stylus,johnnycopperstone/stylus,jangarcia/stylus,PeterWangPo/stylus,Zagorakiss/stylus,rstacruz/stylus,silvaemota/stylus,peterkinalex/stylus-1,cemoulto/stylus