Fixes #258 - Enable multiple values support for header Identifier:identifier

[lsitu]

Fixes #258

Local Checklist

• [x] Tests written and passing locally?
• [x] Code style checked?
• [x] QA-ed locally?
• [x] Rebased with master branch?
• [ ] Configuration updated (if needed)?
• [ ] Documentation updated (if needed)?

What does this PR do?

Added multiple values support for Identifier:identifier field.

Why are we doing this? Any context of related work?

References #251, 258

@ucsdlib/developers - please review

[lsitu]

@ucsdlib/developers Can anyone review and merge the PR as well while @mcritchlow is out? Thanks.

[lsitu]

Thanks @VivianChu for your review and merging of the PR.