Existing metadata overwritten by new uploads

[NickolausDS]

Some data we probably want to keep is overwritten on multiple uploads/registers.

For example, given the creator name 'John' set manually into a metadata.json file and uploaded with pilot upload -j metadata.json foo.txt /. On a second upload without the metadata, the creator name will be overwritten by the pilot user, for example pilot upload -u foo.txt / will overwrite the creator from 'John' to 'Frank', if 'Frank' was the logged in pilot user doing the upload.

We probably don't want to overwrite those fields unless they are missing or specifically specified in another metadata.json file.