Makes DboSource less string dependent.

The reason behind this is that my queries aren't becoming SQL strings, but PHP ([ReQL](https://github.com/danielmewes/php-rql)) objects. I still want to use the higher level CRUD methods from `DboSource` and just implement/override everything underneath that generates the SQL queries. `renderStatement()` is a good example of that.

So I moved the trimming out of the `DboSource::read()` and into `DboSource::renderStatement()` where it belongs. Then `DboSource::read()` can have all sorts of queries. Unfortunately some tests where expecting the trailing whitespace. In my opinion incorrect and so I removed the expected whitespace.