Fix illegal invocation for FormData proxy

[scrhartley]

Description

Currently the form data proxy has special handing for if the return value is a function,
but not in the case where the key is a symbol. This omission can cause an error to be thrown.

Fixes constructor for URLSearchParams:

element.addEventListener('htmx:configRequest', function(event) {
    let params = new URLSearchParams(event.detail.parameters);
    console.log(params.toString());
});

If I was implementing this from fresh then I would use the more concise:

if (typeof result === 'function') {
    return result.bind(formData)
}

but instead I've replicated the existing approach:

if (typeof result === 'function') {
  return function() {
    return result.apply(formData, arguments)
  }
}

Corresponding issue: #2995

Testing

Added a new test to the existing ones for parameters.

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

[1cg]

thank you!