Fix 1103 by evaluating hx-vals with this referring to the element

[maekoos]

Description

Fixes #1103 by evaluating hx-vals attributes with this referring to the current element.

Example:

<div hx-post="/vars" hx-vals="javascript:{ ...this.dataset, }" data-name="a name" data-hello="world"></div>

evaluates to { "name": "a name", "hello": "world" } since this refers to the div with hx-vals attribute.

Corresponding issue: #1103

Testing

Added a test in test/attributes/hx-vals.js. This tests that this acually refers to the current element.

Checklist

• I have read the contribution guidelines
• I have targeted this PR against the correct branch (master for website changes, dev for
  source changes)
• This is either a bugfix, a documentation update, or a new feature that has been explicitly
  approved via an issue
• I ran the test suite locally (npm run test) and verified that it succeeded

Edit: May also fix #1658