Protect specific metadata

[frafra]

It would be nice to be able to retain some previous metadata fields instead of dropping them; this would allow adding additional metadata from the CKAN interface on harvested resources, without risking losing them.

ckanext-harvest uses package update, but a switch to use package_patch instead should be pretty easy to implement. Otherwise, a list of protected metadata could be provided, so they can be copied over if needed.

[frafra]

Here is where the package is updated: https://github.com/ckan/ckanext-harvest/blob/e2e8acbb6d87d44663e92c5ff5cbd67401588c2d/ckanext/harvest/harvesters/base.py#L324-L326