Support updating nested fields with dot notation

[zhezhang77]

Fix #146 Unable to update nested fields with custome mask in updateDocument()

1. Add a flag 'nestedField' in updateDocument() to control if treating fields with dot notation as nested fields or single fields. If it is not set or set to false, the library will keep the original behaviour.
2. Fields in the updater will be merged based on their inserted order
3. Add 3 new tests to for the new flag