Enums don't seem to serialise properly

```
import graphene

class MyEnum(graphene.Enum):
    Foo = 0
    Bar = 1

class Q(graphene.ObjectType):

    foo = graphene.Field(MyEnum)
    def resolve_foo(*args, **kwargs):
        myenum = MyEnum.get(1)
        print(myenum)
        return myenum

schema = graphene.Schema(Q)
print(schema.execute(
    """
    {foo}
    """).data)
```