Add test for DataSet.empty_data_set() method

See https://www.pivotaltracker.com/story/show/72073020
Refactor `DataSet.post` to use 3 helpers methods

As the upcoming `DataSet.empty_data_set` is going to do basically the same
thing.
Implement `DataSet.empty_data_set`

As part of https://www.pivotaltracker.com/story/show/72073020

[#72073020]
Merge pull request #32 from alphagov/support-empty-data-set

Implement `DataSet.empty_data_set`