Fix the deposit XML parser to have multiple authors in json output

It seems that the rest_framework_xml.parsers do not support parsing a list of elements from xml to json. Few hot-fixes available:

• https://github.com/jpadilla/django-rest-framework-xml/issues/11
• https://github.com/jpadilla/django-rest-framework-xml/pull/15

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Migrated from T1131 (view on Phabricator)