Non-provided columns filled with NULL, conflicts with default UUID

I have a field `pkey` which defaults to a generated uuid as follows:

`pkey UUID NOT NULL DEFAULT uuid_generate_v1(),`

I think the plugin is autofilling any missing column with NULL:

```
2020-08-07 17:14:28 -0400 [error]: #0 Got deterministic error again. Dump a record error_class=ActiveRecord::NotNullViolation error="PG::NotNullViolation: ERROR: null value in column \"pkey\" violates not-null constraint\nDETAIL: Failing row contains (null, 2016-06-22 19:10:25, [\"a\", 3], cool_model, 2.0, 0.5).\n: INSERT INTO \"fluentd_test\" (\"pkey\",\"my_datetime\",\"model_data\",\"ml_model_name\",\"model_version\",\"score\") VALUES (NULL,'2016-06-22 19:10:25','[\"a\",3]','cool_model','2.0',0.5)" record=#
```

The INSERT statement could omit the pkey field and the NULL statement and the database would auto-fill with `uuid_generate_v1()`, so the message would not have to contain uuid.

The issue names no source file or test. Start by tracing the SQL output plugin's ActiveRecord INSERT path and reproducing issue #95 with a PostgreSQL UUID default; done means an omitted pkey allows the database default to populate it, with a regression test covering the behavior.

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