Lexer - Possibility to pass invalid characters eg: (null byte)

Even tho lexer check if input data do not contains some invalid characters https://github.com/graphql-go/graphql/blob/v0.7.9/language/lexer/lexer.go#L235
It is possible to use them - Just by sending them as "plain text"
For example: inputString `"ABC\u0041"` instead of being interpreted as `"ABC\u0041"` is changed to `"ABCA"`

Example code:

```golang
func TestReadString(t *testing.T) {
input := `mutation{
requestRefund(input:{
clientMutationId:"2"
nr:"6849905030\u0041"
}){
clientMutationId
}
}`

inputSource := source.Source{Body: []byte(input)}
token, _ := readToken(source.NewSource(&inputSource), 64)

if token.Value != `6849905030\u0041` {
t.Fatal("token incorrect")
}
}
```

https://github.com/graphql-go/graphql/blob/v0.7.9/language/lexer/lexer.go#L281
Probably it require this same check as here https://github.com/graphql-go/graphql/blob/v0.7.9/language/lexer/lexer.go#L235