Meltano

<https://github.com/meltano/sdk/pull/3498|#3498 refactor: Set default `User-Agent` even if user overrides `http_headers`>
Pull request opened by <https://github.com/edgarrmondragon|edgarrmondragon>
• Extracted from <https://github.com/meltano/sdk/pull/3483|#3483>

## Summary by Sourcery

Bug Fixes:

• Preserve the SDK's default User-Agent header when users override http_headers so requests consistently include a User-Agent value.
<https://github.com/meltano/sdk|meltano/sdk>

<https://github.com/meltano/sdk/pull/3498|#3498 refactor: Set default `User-Agent` even if user overrides `http_headers`>
Pull request ready for review by <https://github.com/edgarrmondragon|edgarrmondragon>
<https://github.com/meltano/sdk|meltano/sdk>

<https://github.com/meltano/sdk/pull/3498|#3498 refactor: Set default `User-Agent` even if user overrides `http_headers`>
Pull request merged by <https://github.com/edgarrmondragon|edgarrmondragon>
<https://github.com/meltano/sdk|meltano/sdk>