better type for Record.toJS

[jdeniau]

Record.toJS loose the Record type and return something like:

Actually the following code:

Record({ keyA: 'a', keyB: 'b' });

will return this type.

{
    keyA: unknown,
    keyB: unknown,
}

This PR implement the DeepCopy functionnality to handle the deep convertion made by toJS

It will now return this:

{
    keyA: string,
    keyB: string,
}

It does handle recursive types:

type NestedObject = Record({ map: Map<string, string>(); list: List<string>(); set: Set<string>() });
// type will be : { map: { [key: string]: string }; list: string[]; set: string[]; }

[leebyron]

This is so good