Previously, if a document contained two YAML metadata blocks
that set the same field, the conflict would be resolved in favor
of the first. Now it is resolved in favor of the second (due to
a change in pandoc-types).
This makes the behavior more uniform with other things in pandoc
(such as reference links and `--metadata-file`).
When multiple YAML metadata blocks are used, and two define
the same field, the value defined first takes precedence,
according to the manual. This was changed briefly in
ba3ee62323. This commit
reverts to the original behavior and adds a test case.
