Enforce "multivalued" schema attribute
Enforce "multivalued" schema attribute

When setting a field on a document (eg, during indexing or migrations),
raise an error if multiple values are supplied for a single-valued
field.

When returning documents, always return multi-valued fields as arrays,
and single-valued fields as not-arrays.

theodi/rummager,alphagov/rummager,theodi/rummager,alphagov/rummager