pandoc-server: citeproc parameter doesn't work as documented

[jgm]

According to the documentation,

citeproc: true

sets up citation processing. However, what is really needed (currently) is

filters: ["citeproc"]

Think about whether to change the server or change the API.
(I lean towards changing the server so that it accepts citeproc: true, especially since filters are not generally allowed.)