Fix for encoding error enrolling new nodes

[javuto]

Fix for an unexpected issue storing the raw enrollment bytes coming from osquery. GORM uses the type bytea for PostgreSQL and in golang that is translated to json.RawMessage which is an alias of []byte. In some cases the content was triggering an encoding error that would prevent nodes to be enrolled:

handlers.go:284: error creating node Create failed to encode args[20]: unable to encode json.RawMessage{...} into binary format for bytea (OID 17): cannot find encode plan

Using just an string, (TEXT in pg) solves the issue.