provider instance specification doesn't set description

[matt-beanland]

Diffo.Provider.Instance.Specification's defstruct is [:id, :name, :type, :major_version, :category]

So the description is not passed to upsert_specification and is lost before create_specification

[matt-beanland]

Added missing test of specification description round trip. Added description to struct. Fixed some tests which now have description coming through in the JSON.