This commit completes the XML response handler and adjusts the output
format for all resources to include a top-level property that encloses
the response. This is more consistent with the multi-resource (e.g. GET
/orders) response.
Part of #4165
Based on WP_JSON_Server, this class add response handling based on
endpoint suffixes (.json or .xml) or the ACCEPT header. It also removes
some unneeded functionality and renames filters so as to not conflict
with users who may have WP-API installed already. Finally, instead of
no authentication required by default, authentication is always
required unless specifically disabled via filters.
Coen Jacobs
Max Rice
Scott Basgaard
Gerhard