Reference:  parse date-parts with numbers instead of strings.
citeproc-tests: Skip bibliography-header entries.
Reference/JSON:  Allow more fields to be numbers.
citeproc-tests improvements.
Reference/JSON: Handle heterogenous string/number lists for date-parts.
citeproc-tests: Allow 1, 0 to be parsed as Bool.
Reference/JSON: don't require "id".
Fixed citeproc-tests.