fix side effect from queryset.no_dereference

[bagerard]

The queryset.no_dereference() method was having side effect on the class Field (descriptor) _auto_dereference attribute. Meaning that existing instances could be impacted by a call to no_dereference and suddenly stop dereferencing. See below:

class User(me.Document):
    name = me.StringField()
    
class Organization(me.Document):
    name = me.StringField()
    user = me.ReferenceField(User)

user = User(name='John').save()
Organization(name='GitHub', user=user).save()

org = Organization.objects.first()   # fetch an instance before the no_dereference call
_ = Organization.objects.no_dereference().first() # alters the _auto_dereference of the class field (i.e the ReferenceField descriptor) 
print org.user    # DBRef('user', ObjectId('5b9ec11d4ec5dc58df8290ba')) <-- SHould have been dereferenced

Fixes #1677 (more info in the issue)

Note that:

• this does not fix the no_dereference context manager
• CachedReferenceField & GenericReferenceField suffers from the same bug. I'll fix those in a separate PR