Change completeness metrics code to use symbols

The completeness metrics code depended on string keys as well. Instead
of parsing the data again I decided to symbolize the CKAN schema.

Signed-off-by: Konrad Reiche <konrad.reiche@gmail.com>
Fix failing tests

The tests I have written so far failed due to different reasons. This
commit fixes that.

Signed-off-by: Konrad Reiche <konrad.reiche@gmail.com>