Allow merging nullable and non-null fields

It is widely believed that, in the process of schema evolution, changing field type from `T` to `T!` is not a breaking change.
A counter-example is given below (a like of it was encountered in a real-life application).

Consider the following [partial] schema
```graphql
union ExternalResource = LinkedResource | ResourceCitation

type LinkedResource {
title: String!
link: String
}

type ResourceCitation {
citation: String!
link: String
}
```
and this selection set
```graphql
{
... on LinkedResource {
link
}
... on ResourceCitation {
link
}
}
```
The selection set is valid (`FieldsInSetCanMerge` returns `true` on it).
Suppose the schema evolves:
```graphql
type LinkedResource {
title: String!
link: String!
}
```
The selection set given above is now invalid, since `SameResponseShape` (step 3a) returns `false`.

Since `T!` is naturally embedded in `T`, it seems reasonable to allow such 'compatible' fields to be queried without forcing aliasing upon API clients (and possibly causing a breaking change where, semantically, there is none).

The problem looks related to these recently opened issues:
https://github.com/graphql/graphql-spec/issues/1171
https://github.com/graphql/graphql-spec/issues/1169