Fix spine start_date

[b-luu]

On PostgreSQL, the start_date of the date_spine as string causes an error.

Are you a current Fivetran customer?
Brice Luu, Freelance Data Engineer, currently implementing fivetran+dbt at Welcome to the Jungle.

What change(s) does this PR introduce?
Explicitly casting as date the start_date of the date_spine dbt_utils function.

Does this PR introduce a breaking change?

• Yes (please provide breaking change details below.)
• No (please provide explanation how the change is non breaking below.)
  It just fixes the fivetran_log__connector_daily_events model (doesn't work without this)

Is this PR in response to a previously created Issue

• Yes, Issue [link issue number here]
• No

How did you test the PR changes?

• CircleCi
• Other (please provide additional testing details below)
  Manually ran dbt run --select fivetran_log__connector_daily_events
  Output:

Running with dbt=0.21.0

Found 28 models, 48 tests, 1 snapshot, 1 analysis, 429 macros, 0 operations, 0 seed files, 36 sources, 0 exposures

10:21:34 | Concurrency: 2 threads (target='ods')
10:21:34 | 
10:21:34 | 1 of 1 START table model dbt_kian.fivetran_log__connector_daily_events [RUN]
10:21:41 | 1 of 1 OK created table model dbt_kian.fivetran_log__connector_daily_events [SELECT 399 in 7.39s]
10:21:41 | 
10:21:41 | Finished running 1 table model in 8.48s.

Completed successfully

Done. PASS=1 WARN=0 ERROR=0 SKIP=0 TOTAL=1

Select which warehouse(s) were used to test the PR

• BigQuery
• Redshift
• Snowflake
• Postgres
• Databricks
• Other (provide details below)

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