fix: edge case where empty value overwrite existing value

[xprotocol-bot]

Reason for This PR

We found another edge case where http package can potentially wrongly overwrite data

Let's say that we have a form like this:

form := url.Values{}
form.Add("name[]", "name1")
form.Add("name[]", "name2")
form.Add("name[]", "name3")
form.Add("name", "")

Because iterating over map in Go is random so this case might not happen all the time. But when it happens,
data under name key will be empty string which in our case removes all the relationships in Laravel backpack.

I also refactor the test cases to be shorter and with better description.

Description of Changes

Fix one more edge case where empty value can potentially overwrite existing value.

License Acceptance

By submitting this pull request, I confirm that my contribution is made under
the terms of the MIT license.

PR Checklist

• All commits in this PR are signed (git commit -s).
• The reason for this PR is clearly provided (issue no. or explanation).
• The description of changes is clear and encompassing.
• Any required documentation changes (code and docs) are included in this PR.
• Any user-facing changes are mentioned in CHANGELOG.md.
• All added/changed functionality is tested.

[rustatian]

Hey 👋🏻, great work @tungfinblox, thanks 👍🏻
I'll have a look tomorrow ⚡

[rustatian]

Everything else looks good (mmm generics 😆), nice job, thanks 👍🏻

[xprotocol-bot]

Everything else looks good (mmm generics laughing), nice job, thanks 👍🏻

Thank you. I updated it :D