Created DataMixin.
Created DataMixin.

We can subclass a single test class with a bunch of tests for storing
data; this allows us to not write the same tests for each type of data
we want to store.

datreant/datreant,andreabedini/datreant,datreant/datreant.core,dotsdl/datreant,datreant/datreant.core,datreant/datreant,datreant/datreant.data