move #default_mask to essence_picture.rb
fix cropping test

In the test for the ```#crop``` method, ```@picture.default_mask``` was stubbed
with values that cannot possibly arise using the current ```default_mask```
method. This commit sets them to the dimensions of the full image.
Change :x, :y to :width, :height and make sure #default_mask returns Int

OK, so mask[:width] is infinitely more readable than mask[:x].

Plus: added some test to check for correct resizing and found out it
didn't resize correctly. Refactored again to make absolutely clear
what actually happens.

Also: #default_mask has to return only integer values in the Hash.