Unable to update a metadata if the passed object doesn't have certain attributes

[Guts]

To reproduce:

from isogeo_pysdk import Isogeo, Metadata


[..authentication steps..]

# get the metadata to update
metadata_to_update = isogeo.metadata.get(metadata_id=the_uuid_of_the_metadata_to_update)

# simulate a newly empty object
newly_metadata = Metadata(_id=metadata_to_update._id)

# just edit the title
newly_metadata.title = "OMG I've been updated!"

# try to update
isogeo.metadata.update(nely_metadata)
>>> 500: Internal Server Error

Let's add a warning and document that.