Allow detached rulesets as mixin argument defaults

[calvinjuarez]

This fixes #2496, allowing detached rulesets to be declared as mixin argument defaults.

For example:

.mixin(@size: 30px; @padding: 30px; @additions: {}) { // the "@additions: {}" used to error
  height: @size;
  width: @size;
  padding: @padding;
  @additions();
}

.no-additions {
  .mixin();
}
.additions {
  .mixin(@additions: {color: white;});
}

compiles to

.no-additions {
  height: 30px;
  width: 30px;
  padding: 30px;
}
.additions {
  height: 30px;
  width: 30px;
  padding: 30px;
  color: white;
}

[lukeapage]

thanks