Fix #119: Fix constructing more complex object with arrays

[meex28]

Fixing the bug #119

Description

Fixed incorrect array handling when combining complex hx-vals with json-enc. Previously, when processing:
<button hx-ext="json-enc" hx-vals='{"Id":1,"Items":["abc","xyz"]}'> ...</button>

The encodeParameters function gets as input FormData object like:

• Id: '1'
• Items: "abc"
• Items: "xyz"

When retrieving types from hx-vals, for each entry with an Items key, whole array was added to the result object:

{
    Id: 1,
    Items: [ ["abc", "xyz"], ["abc", "xyz"] ]
}

Instead of:

{
    Id: 1,
    Items: ["abc", "xyz"]
}

To change this behaviour, the function now first groups FormData parameters by keys, and then maps raw values to types from hx-vals.

Testing

In the test handles hx-vals properly added an array-type value ('array': ['text', 123, {'key': 'value'}]) and expects it to to be properly included in the body.

Checklist

• I have read the contribution guidelines
• I ran the test suite locally (npm run test) and verified that it succeeded

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[Telroshan]

Hey, thanks for your work @meex28 !
While I understand the desire to refactor with a more modern JS syntax, I'm not convinced it's necessary to introduce so many tricks (array from, reduce, map, from entries) when all that's needed is to split the overriding with hx-vals/hx-vars values, to a second iteration instead of a value-by-value basis in a single loop.

Hence why I took the liberty to simplify your code to reduce the diff compared to the previous code, I hope you understand!
The idea is that the more the new code looks like the previous one, the easier it is to compare & review (as the diff is smaller).
To each their own of course, but I find a good old foreach way easier to review & process mentally rather than a chain of array/object callback-based functions

Thanks again for your work & patience!