Allow expression vars to override raw parameters

[bencroker]

This PR allow expression vars to override raw parameters.

Before:

<form hx-get="">
    <inpup type="hidden" name="temperature" value="0">
    <button type="submit" hx-vars="temperature: -23">Go sub-zero</button>
</form>

<!-- Submits {temperature: 0} -->

After:

<form hx-get="">
    <inpup type="hidden" name="temperature" value="0">
    <button type="submit" hx-vars="temperature: -23">Go sub-zero</button>
</form>

<!-- Submits {temperature: -23} -->

[bencroker]

@1cg I just found a test called hx-vars do not override inputs, so this is by design?
https://github.com/bigskysoftware/htmx/blob/dev/test/attributes/hx-vars.js#L62-L73

[1cg]

I don't see a compelling reason that it was done this way. Seems like the hx-vars is more local to the call and so it should have precedence.

I'll pull and modify the test.

[bencroker]

Thanks!