fixes for svelte's useForm helper

[edgars-vasiljevs]

This fixes two issues in useForm helper that I found.

• Form fields with objects as values (arrays/objects) has broken reactivity. This is because input data is shallow copied to defaults. Updating nested objects in form updates defaults object as well. Since the current form data is compared to defaults, no changes are detected.
• Broken setStore. Svelte's stores replaces store value with anything it receives from update's callback function. In this case setStore creates completely new object instance every time it's called by returning Object.assign({}, store, ...). Later when form submit fails, this.clearErrors().setError(errors) is called on object instance that request was made from which is NOT the object currently stored in form store hence nothing is cleared.

This should fix form error issue in #1521 #1276

Edit:

Added two more fixes:

• reset function also shallow copies but this time defaults to data. This again breaks nested array/object reactivity.
  - updating defaults to current data after successful request. It's debatable but IMO form defaults should be updated because previous form state is irrelevant after successful submit.

I see that Vue adapters work very similarly to the changes I made.

[taylorotwell]

Hey there - this is on our radar. Hoping to get it merged next week. 👍

[jessarcher]

I've tested this locally with and without this branch and can confirm the issue and the fix.

Thank you!