Fix: Forward registered functions to mixin conditions

<a href="https://github.com/s7eph4n"><img src="https://avatars.githubusercontent.com/u/2829850?v=2" align="left" width="96" height="96" hspace="10"></img></a> **Issue by [s7eph4n](https://github.com/s7eph4n)**
_Saturday Aug 16, 2014 at 23:08 GMT_
_Originally opened as https://github.com/oyejorge/less.php/pull/160_

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Registered functions need to be forwarded to mixin conditions, else they will not be available for "while" statements.

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_**[s7eph4n](https://github.com/s7eph4n)** included the following code: https://github.com/oyejorge/less.php/pull/160/commits_