Project imports were failing with `undefined method each_with_object for
String` because the import was attempting to parse the LFS Batch API and
failing due to the fact that the Content-Type wasn't a supported format
(e.g. application/vnd.git-lfs+json instead of application/json). We now
parse the body as JSON.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/61624