Handle array-like objects when working with checkboxes

[Lustmored]

Q	A
Bug fix?	yes
New feature?	no
Tickets
License	MIT

First of all please take a look at added test cases. I think it shows the purpose of this change clearly.

Background:
Recently when working with forms inside a component I ran into a corner case, where Symfony forms are setting a non-zero-indexed array for the properties of ChoiceType with both multiple and expanded. It breaks live-component model assumption that multiple checkboxes keep their value as an array, which in JavaScript is always zero-indexed.

It happens after a pretty weird set of steps:

• check any option (foo)
• check any other option (bar)
• check the third option (baz)
• uncheck option 'foo' <- Symfony forms store current value as [1 => 'bar', 2 => 'baz']
• uncheck option 'bar' <- at this point current implementation is treating checkboxes as a single value and send back null instead of ['baz'].

This simple change fixes this flow (tested on a real life project). I don't see any correlation with failing tests.

[weaverryan]

Very-well explained and excellent tests - THANK YOU. Check boxes are deceivingly complex