don't need to instanciate new objects to check their constructor

Hi,

I'm not sure if it's a really relevant pull request, the fact is I had done similar modification to what you did in the last months ( https://github.com/JeanSebTr/barista/commit/6d3351b7c44449890ce43e64230b3372d17eaf0f <> https://github.com/kieran/barista/commit/22fd062cf761003a91c69197ae65ab7a77bc06ce ). This one is what it's left of those.

I made modifications and didn't have a chance to really use barista in a project yet, so I forgot about them.