NdbRecordObject does not handle sparse fields

[jdduncan]

If use_mapped_ndb_record = false, the ndb adapter correctly handles sparse fields.

But if use_mapped_ndb_record = true, SPARSE_FIELDS itself shows up as a property:

{ SPARSE_FIELDS: '{"profile_text":"I am your little boot!"}',
tweet_count: 0,
full_name: 'Gaius Julius Casear Germanicus',
user_name: 'caligula',
profile_text: 'I am your little boot!' }