When enum is used as argument, its default value is not validated against enum definition

Hello!

While working on enum support in Ariadne I've found that enums values aren't validated against GraphQL enum when they are defined as fields on GraphQL schema.

Following schema passes validation:

```graphql
enum Role {
ADMIN
USER
}

type Query {
hello(r: Role = ADMIN): String
complex(i: Test = { role: INVALID }): String
complexAlt(i: Test): String
}

input Test {
ignore: String
role: Role = INVALID
}
```

I would've expected schema validators to raise `INVALID`, but instead those fields are quietly excluded from default value passed to field's resolver.