JSON is not returned when setting the 'Accept' header for the URL http://schema.org/.

This can be reproduced here:
curl -H "Accept:application/ld+json" https://schema.org/ -I
which returns a response with:
Content-Type: text/html

This seems to be just the base URL. Other schema.org URLs return JSON, such as:
curl -H "Accept:application/ld+json" https://schema.org/docs/jsonldcontext.json -I

This is causing some tests to fail, such as:
com.github.jsonldjava.core.DocumentLoaderTest#loadDocumentSchemaOrg