Remove empty quotes when category is empty.

When category = ""

Should print "category: " to file, current behaviour prints "category: \"\"".

Latter creates an actual category, with a blank title. All posts created
since this update, without a category explicitly written in `rake`
command, or post directly edited, will belong to this category.

Creating the post:
![screenshot 2014-01-01 20 42 44](https://f.cloud.github.com/assets/318829/1830143/67f45b34-7325-11e3-86fe-3d3e79dcddd9.png)

The post:
![screenshot 2014-01-01 20 43 25](https://f.cloud.github.com/assets/318829/1830142/67ed154a-7325-11e3-988e-b488a2dc41b5.png)

The category:
![screenshot 2014-01-01 20 43 32](https://f.cloud.github.com/assets/318829/1830141/67e960da-7325-11e3-884a-16f0a4fed460.png)