invert() with $weight: 50% is wrong

[nex3]

Dart Sass's definition of the invert() function has the $weight parameter default to 50%, which is wrong (Ruby Sass defaults to 100%). The only reason this doesn't cause huge problems is that it also has a special case to avoid the call to mix() when $weight is 50%, but this means that if $weight is explicitly passed as 50% it gives the wrong result.