Incorrect handling of UnicodeBOM in GraphQL grammar

[gruzdev]

UnicodeBOM must be ignored everywhere in a document not just at the beginning.

[gruzdev]

And this is how Apollo just ignores UTF-8 BOM after opening curly:

$ echo -ne '{"query": "{\xEF\xBB\xBF__typename }"}' | curl -X POST -H "Content-Type: application/json" --data-binary @- http://localhost:4000
{"data":{"__typename":"Query"}}