fix hex bit strings with leading zeroes in postgresql

[jstanley0]

Motivation / Background

Currently when casting a hex string argument for a PostgreSQL bit or bit_varying column, leading zeroes are discarded. This is unexpected, and differs from the behavior of binary strings.

Detail

This Pull Request ensures leading zeroes are preserved, e.g. assigning a bit string column "0x5F" results in the bit string "01011111" instead of "1011111" being sent to Postgres.

Additional information

Checklist

Before submitting the PR make sure the following are checked:

• This Pull Request is related to one change. Unrelated changes should be opened in separate PRs.
• Commit message has a detailed description of what changed and why. If this PR fixes a related issue include it in the commit message. Ex: [Fix #issue-number]
• Tests are added or updated if you fix a bug or add a feature.
• CHANGELOG files are updated for the changed libraries if there is a behavior change or additional feature. Minor bug fixes and documentation changes should not be included.

[jstanley0]

hmm. that test doesn't fail locally and also doesn't depend on the Postgres ActiveRecord adapter afaict