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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