Fixed regression in JSON output for date-parts.
Fixed regression in JSON output for date-parts.

Previous versions of represented dates as:
`"issued": {"date-parts": [[2006]]}`.  pandoc-citeproc 0.14
does instead: `"issued": [{"date-parts": [2006]}]` which is
not expected.

Closes #321.

adunning/pandoc-citeproc,jgm/pandoc-citeproc