Backport get_avatar_url() fix to v1 REST API
REST API: Fix JSONP content-type
REST API: Mitigate possible JSONP flash attacks

see http://miki.it/blog/2014/7/8/abusing-jsonp-with-rosetta-flash/
REST API: Add order statuses endpoint
REST API: Add field restriction to new endpoints
Fix notice introduced in #6084
REST API: ensure consistency in create/edit data formats

A client should be able to GET a resource, edit a single item and PUT
it back to the server without having to perform any additional
transformation. This commit ensures all PUT/POST endpoints adhere to
this standard.
REST API: tweak filter name from #6084
REST API: allow adding/updating order meta

When creating or updating orders, non-protected scalar meta can now be
set.
REST API: Fix edit coupon typos
REST API: allow coupon description in create/edit coupon
Merge pull request #6140 from maxrice/v2-rest-api-tweaks

v2 REST API Tweaks