Serialize form using UrlSearchParams

[daun]

• Switch to native URLSearchParams API to serialize form data before submission
• Fixes Doesn't submit textareas #25
• Simplifies code and improves handling of edge cases
• Changes browser compatibility, so probably worth a major version

[hirasso]

While testing this PR I stumbled upon a possible bug: The form's get parameter isn't being replaced in the URL but instead appended. This leads to this if submitting a form multiple times:

?searched=somtething&searched=another&searched=yet+another

https://test.rassohilber.com/swup-forms-plugin/test/

If you submit the form a few times you can see how the URL grows... and grows... :)

This bug actually already existed before the edits in this branch. Should I take care of fixing this and just push it to this PR as well? Or too messy?

[daun]

@hirasso Right, nice catch!

I've never come across this one as I always set the action attribute, but yes, we should handle it the same as the browser would. The solution is probably to construct the action from window.location.path instead of window.location.href for GET requests, right? I can push a fix later.

[daun]

Looks like that's exactly what browsers do for GET requests: strip everything after the question mark.

[daun]

@hirasso I've fixed the mentioned bug and did some additional cleanup. Ready for review :)