fixed plugin 'date' after pull request #1397 from IQAndreas/cleanup-date

The commit 64ba603 breaks the date display for posts that do not define the date into their YAML front-matter block.
The old code was using self.date to format the date.
You can check how it looks [before](http://axiac.ro/before-cleanup-date/) and [after](http://axiac.ro/after-cleanup-date/) commit 64ba603.

I checked into the source code of Jekyll and found out that, for posts, `self.date` is extracted from the file name of the post then it is overridden by the date specified in the YAML front-matter block, if any. The pages do not have this property.