The working scenario is first detailed below:

URL -> http://localhost:8181/restconf/operations/toaster:make-toast
Headers -> Content-type: application/xml
Body ->
<input xmlns="http://netconfcentral.org/ns/toaster">
<toasterDoneness>1</toasterDoneness>
<toasterToastType>wheat-bread</toasterToastType>
</input>

This results in HTTP 204, success.

However, if you add a trailing slash to the URL it does not:
URL -> http://localhost:8181/restconf/operations/toaster:make-toast/
Headers -> Content-type: application/xml
Body ->
<input xmlns="http://netconfcentral.org/ns/toaster">
<toasterDoneness>1</toasterDoneness>
<toasterToastType>wheat-bread</toasterToastType>
</input>

This results in HTTP 400, Bad Request. The trailing slash should not result in HTTP 400. This is fixed in stable/lithium, but is a bug in stable/helium.