Required for REST URLs which require data to be populated on call.
For example, PUT admin/campaigns/set/:id/publish to publish a campaign.
At the time when createEndpointFetcher() is called, the ":id" parameter can not be resolved,
since the "data" argument is passed in once the returned function is evaluated (since the component using it).
Since the whole idea of createEndpointFetcher() is that components shouldn't know about the backend internals,
its not the right place to interpolate urls.
I'm sending through any payload data in addition to replacing URLs, to keep the API simple for the component.
Again, it shouldn't worry about the presence URL interpolation. The trade off here is that data will be submitted
both in the payload (in case of a POST, PUT or DELETE request), as well as (optionally) in the URL.
SilverStripeBackend now has a createEndpointFetcher method, described
in its docblock in more detail.
It’s based the asset-admin code for apiCallerFromEndpoint(). It’s
refactored and generalised into framework in order to provide a
general-purpose API for tying Javascript components to backend APIs.
@todo: tests
FIX: Ensure post has correct content-type header.
Exporting silverstripe-backend lets other modules (such as asset-admin)
use it.
In addition, the Content-type header of a post request of URL-encoded
data was being set to text/plain by default, which isn’t correct and
stopped PHP from interpreting it.
