Fix object comparison

[vinicius-lino-figueiredo]

Make model.compareThings actually work for objects; model.compareThings would
only check field values and object length, ignoring object keys. Now it works
for those specific cases where it would not before.

In this code:

const db = new Datastore()
db.ensureIndex({ fieldName: "uniqueField", unique: true }, function(err) {
  db.insert({ uniqueField: { one: 'a' } }, function() {
    db.insert({ uniqueField: { another: 'a' } }, function(err) {
    })
  })
})

the second insert would return an error, when comparing {one:1} and {another:1},
the old function would:

1. List keys from both fields (['one'], ['another']);
2. Sort them (no change);
3. Compare the respective values ('a' and 'a')
4. Since they are all equal, compare lengths (1 and 1) and return result (legths
   are equal too).

Now, if length comparison does not return a difference, we compare the key list.