Pass bytes instead a dict to origin_metadata_add.
swh-storage v0.0.191 unexpected allowed a dict under some circumstances, but it should always be bytes (and v0.0.192 will enforce this).
Test Plan
Tests would start failing when swh-storage!401 (closed) is landed and released (ie. v0.0.192)
Migrated from D3169 (view on Phabricator)