Fix: Make union_relations include/exclude case insensitive

[jeremyyeo]

This is a:

• documentation update
• bug fix with no breaking changes
• new functionality
• a breaking change

All pull requests from community contributors should target the main branch (default).

Description & motivation

Fixes #578.

Checklist

• I have verified that these changes work locally on the following warehouses (Note: it's okay if you do not have access to all warehouses, this helps us understand what has been covered)
  • BigQuery
  • Postgres
  • Redshift
  • Snowflake
• I followed guidelines to ensure that my changes will work on "non-core" adapters by:
  • dispatching any new macro(s) so non-core adapters can also use them (e.g. the star() source)
  • using the limit_zero() macro in place of the literal string: limit 0
  • using dbt_utils.type_* macros instead of explicit datatypes (e.g. dbt_utils.type_timestamp() instead of TIMESTAMP
• I have updated the README.md (if applicable)
• I have added tests & descriptions to my models (and macros if applicable)
• I have added an entry to CHANGELOG.md

[jeremyyeo]

@dbeatty10, added your suggested test but I tweaked it a little to actually include upper/lowercase in the exclude param... wasn't sure if the test previously would have had that or not 😁

[joellabes]

As of #624 and #661, you can pass none into source_column_name to suppress the column from being created, if you'd rather.

[joellabes]

@jeremyyeo I'm going to merge this into the v1 branch, if (in a separate PR) you wanted to change the integration test to exclude the source column it'd make things slightly tidier, but I won't hold this up for that!