The guarded namespace can use when the mixin is invoked

What: fix https://github.com/less/less.js/issues/3407

Why: If a namespace has a guard, mixins defined by it are used only if the guard condition returns true.The behavior of the variables should also be consistent.

How: In matchcondition we should let the condition evaluate.

Checklist:

[x] Documentation
[x] Added/updated unit tests
[x] Code complete

less / less.js

The guarded namespace can use when the mixin is invoked #3725