Fix WDSL typo for uploading WSDL attachments

Fix WDSL typo for uploading WSDL attachments
Merge pull request #1848 from alphagov/upload-wsdl-attachments

Fix WDSL typo for uploading WSDL attachments
content_id must be lowercase
Allow image caption to be null

There is a potential discussion to be had about whether we should exclude keys
when the value is nil on optional fields, but that's for another time.
Merge pull request #1852 from alphagov/case-study-json-schema

Use a JSON schema for validating case studies
Add organisation guids to case studies links hash.
Add organisations to case_study JSON schema.
Push all case studies to publishing API again.
Verify case study with orgs JSON is valid

Just some extra safety to make sure we're producing valid JSON.