Allow to include blank values in the form request

Hello,

Thank you for wiselinks, it is really useful and flexible!

I need to be able to post all form fields, even when they're blank, so I can clear out their corresponding attribute on the server.

I believe you temporarily disable inputs and selects with an empty value when serializing the form to filter out blank values. So I wrote a quick workaround to allow not filtering out those blank values by adding a ```data-include-blank="true"``` attribute to the form tag.

Why do you filter out blank values, or did I get it wrong?

I wrote this pull request to start the discussion to understand the best way to address posting blank values with wiselinks. I can rework this pull request in a cleaner way if you're interested.

Cheers,

Mathieu