Protect the `.fields' variable state from partial-initialization timing
issues.
As gorm warms state and makes `.GetStructFields' calls, they lead to a long
deferred function in model_struct.go. Then the deferred function calls
`Scope.Fields', leading to another `.GetStructFields' call.. cyclically.
Once information is cached the cycling ends.
This extra rule evicts incorrect/suspiciously short fields information.
Symptom of incorrect internal state ocurring can manifest as invalid
insert SQL statement errors generated during `DB.Save', e.g.:
INSERT INTO "your_table" DEFAULT VALUES RETURNING "your_table"."id"
This is why we refresh if nil or only a single field (usually is the ID field).
Included is a unit-test which triggers the scenario and prove that it is
fixed.
