Remove sanitizing __ types

[maximgeerinck]

When using graphql, we rely on __typename for identifying the correct types in a union type.

imagine i have the following interface

interface Animal {
    id: ID!
}

type Crawler implements Animal {
    legs: Int
}

type Swimmer implements Animal {
    fins: Int
}

we would return a type of the form union SomeAnimal = Crawler & Swimmer where we rely on __typename to correctly display some information on the dashboard.

With this proposal I ask to not filter out __ fields.