Private fields and `db_field` arg’

Hello,

I’m playing with private fields with MongoEngine and Graphene-Mongo like this:
```python
class MyDocument(Document):
# define a “private” field (with underscore)
# but set the database field without the underscore:
_private = StringField(db_field="private")

class MyType(MongoengineObjectType):
class Meta:
model = MyDocument
```

And… the generated `MyType` has a field named `Private` (not `private`).

That make me ask 2 questions:
* It seems the conversion snake_case to camelCase does not make any special case for leading underscore: is it normal?
* Why not consider the database field?

Start at the MongoengineObjectType field-generation and snake_case-to-camelCase conversion entry points, then trace how the leading-underscore field and its db_field value become the GraphQL name. Done means the intended naming behavior is established and covered by a regression test for the shown MyDocument and MyType example.

Scritto dal modello di indicizzazione a partire dal testo della issue.