ES schema DOI (and other identifiers) should be case-insensitive

[bnewbold]

DOIs should be considered case-insensitive, but this query doesn't work as expected:

https://scholar-qa.archive.org/search?q=10.4039%2FEnt4512-1
https://scholar-qa.archive.org/search?q=10.4039%2Fent4512-1

Working around direct DOI queries should be easy enough in the fatcat-scholar code (just lower-case before running the query), but it would probably be good to make the search field itself case-insensitive.

[bnewbold]

Worked around this particular case by lower-casing direct DOI queries, but really the identifier field should be case-insensitive at the ES schema abstraction layer. Keeping this issue open until that is resolved.