hx-headers inheritance cannot be disabled

The docs note that:
`hx-headers is inherited and can be placed on a parent element`.

However, `hx-headers` does not seem to respect inheritance settings via `hx-disinherit="hx-headers"` or ``.

The source at htmx.js:3514 uses

```getValuesForElement(elt, 'hx-headers', false, headers)```

rather than `getClosestAttributeValue` which is used by other disinheritable attributes.

Any content swapped into a parent element using `hx-headers` will also inherit the same headers. Current workaround is to add an additional containing `div` that unsets the headers.

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