Revert "api.metacpan looks to no return "field" but rather "_source" now"

This reverts commit b0e6e6250df30528b0d5a9a802fca298eb0f28a3.  The
switch in data format was due to a Fastly caching layer ignoring the
body on GET requests, and thus the set of requested fields.  Fastly has
been rolled back until ElasticSearch has been modified to send POST
requests.