fix(posts-query): specific posts getting

[adekbadek]

All Submissions:

• Have you followed the Newspack Contributing guideline?
• Does your code follow the WordPress' coding standards and VIP Go coding standards?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Changes proposed in this Pull Request:

#1083 re-instated issue fixed by #857. There's some confusion around "post type" attribute's casing in Newspack_Blocks::build_articles_query method – if post_type is supplied in the $attributes argument (instead of postType), it will be overridden with the default value (['post']).
This PR ensures that the call to the API always uses the camel-case postType (and not snake-cased post_type).

This confusion arises from the fact that postType is the block attribute, and post_type is the API call param.

How to test the changes in this Pull Request:

1. Observe that the issue described in fix: show all saved post types in specific posts field #857 is reproducible on master
2. Switch to this branch, observe the issue is not reproducible

Other information:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes, as applicable?
• Have you successfully ran tests with your changes locally?

[matticbot]

🎉 This PR is included in version 1.47.1 🎉

The release is available on GitHub release

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