Add a test for hiding fields on a single document
Add a test for hiding fields on a single document

Demonstrates the behaviour when an individual document has been flagged
for having a subset of its fields hidden. This test is currently
failing, which is expected because the feature hasn't been implemented
yet.

mysociety/popit-api,Sinar/popit-api,mysociety/popit-api,ciudadanointeligente/popit-api,Sinar/popit-api