Fix parsing of complex arguments

Given a complex mixin call like this:

````scss
@include multi-backgrounds(url('sprite.png') 0 -276px no-repeat, (320deg, #e2d0ec, lighten(#e2d0ec, 10%)), lighten(#e2d0ec, 5%));
````

The current master branch fails with an Exception about missing parameters. The offending commit is 24a8813 

This pull request adds a test for the failure according to the commit message of 24a8813 and fixes as well as adds tests for the currently failing argument parsing.