Make `update` method send `PATCH`, and add separate `replace` method which can send `PUT`

[asgeo1]

Addresses issue from https://github.com/andreibondarev/weaviate-ruby/issues/36, so user can either PATCH or PUT objects in Weaviave class as required.

[andreibondarev]

@asgeo1 Would you please update the specs as well? Thank you!