Fix form data with show curl before try

[JochenHeizmann]

I had some problems with RapiDoc with multipart/form-data Requests in combination with generating the CURL Requests.

First, it didn't handle the items I enter in a <tag-input> component correctly.

It also resets the items each time when I clicked "TRY" to send the request (when show-curl-before-try is set to 'true').

Finally, it also didn't live update the CURL request.

Besides my changes to address those issues, I've also added an example specs to try it out (docs/specs/form-data-test.json).

FYI: I've based this pull request on the v9.3.8 because that was the version I used for my project.