Removed possible reactivity between data and variables

[indykoning]

This prevents a bug where the unfinished address form gets cleared if the e-mail address is filled in the meantime.

Steps to reproduce:

1. go to https://demo.rapidez.io
2. Enter the onestep checkout
3. Fill in your firstname and lastname before filling in an e-mail
4. fill in an e-mail
5. Firstname and lastname are cleared

This only happens once, if you enter the firstname and lastname again. And amend your e-mail address it no longer changes.

It seems to be that this.data = this.variables causes unintended reactivity between some fields, circumventing the variables watcher. Causing it to see a change while externally no changes have occurred.

This PR removes any possible reactivity between this.variables and this.data only allowing the variables watcher to sync data from variables to data