These are things like non-breaking spaces, "replacement" characters,
etc that add nothing to the metadata and often cause errors during
parsing or displaying in a UI.
In this case it fixes occurences of invalid multi-value separators.
DSpace uses "||" to separate multiple values in one field, but our
editors sometimes give us files with mistakes like "|". We can fix
these to be correct multi-value separators if we are sure that the
metadata is not actually using "|" for some legitimate purpose.
I'm only concerned with validating issue dates here. In DSpace they
are generally always YYYY, YYY-MM, or YYYY-MM-DD (though in theory
they could be any valid ISO8601 format).
This also checks for cases where the date is missing and where the
metadata has specified multiple dates like "1990||1991", as this is
valid, but there is no practical value for it in our system.
For now only test fixes because they return changed data. I'm not
sure how to test the checks, because they don't return data and I
can't modify them to return boolean values without breaking the app.