GraphQL: names can contain invalid characters, and crashes GraphiQL

[dunglas]

API Platform version(s) affected: 2.6

Description

How to reproduce

git clone https://github.com/api-platform/core
cd core/tests/Fixtures/app
php -S localhost:8000 -t public/
open http://localhost:8000/graphql/graphiql

Possible Solution
Encore or replace invalid chars.