Bug: TextSymbolizer.horizontal-alignment - always left when using expressions

I found a bug, expressions in TextSymbolizer horizontal-alignment do not work, they will always be aligned to the left. I'm using the current master branch (commit 8b2243336b53a94494c461cf016e2ffa1ed05c20).

This pictures are created when I run the test:
Agg:
![formatting-expr-horizontal-alignment-500-100-1 0-agg](https://cloud.githubusercontent.com/assets/249012/3776342/ff589f0c-1952-11e4-981d-8ee9e5588cfe.png)
Cairo:
![formatting-expr-horizontal-alignment-500-100-1 0-cairo](https://cloud.githubusercontent.com/assets/249012/3776343/ff5eea4c-1952-11e4-90f0-47f3f71f766b.png)