Using toaster RPC as an example, POST XML to the REST endpoint fails while the equivalent POST using JSON succeeds.

XML input:
Endpoint: http://localhost:8181/restconf/operations/toaster:make-toast
Content-Type: application/xml
Body:
<input>
<toasterDoneness>1</toasterDoneness>
<toasterToastType>wheat-bread</toasterToastType>
<foo>fooValue</foo>
</input>

Status: 400 Bad Request
{"errors":{"error":[

{"error-type":"protocol","error-tag":"malformed-message","error-message":"Error parsing input: Unknown child(ren) node(s) detected, identified by: toasterDoneness, in: container input"}

]}}

When I try using JSON:
Endpoint: http://localhost:8181/restconf/operations/toaster:make-toast
Content-Type: application/xml
Body:
{
"input" :

{ "toaster:toasterDoneness" : "1", "toaster:toasterToastType" : "wheat-bread", "toaster:foo" : "fooValue" }

}
Status: 200 OK