Do not wrap send_join errors on /v1/send_join (#1143)

* Do not wrap v1 send_join errors in [code, body]

* Don't wrap errors

federationapi/routing/routing.go

@@ -18,6 +18,7 @@ import (
 	"net/http"
 
 	"github.com/gorilla/mux"
+	"github.com/matrix-org/dendrite/clientapi/jsonerror"
 	eduserverAPI "github.com/matrix-org/dendrite/eduserver/api"
 	federationSenderAPI "github.com/matrix-org/dendrite/federationsender/api"
 	"github.com/matrix-org/dendrite/internal/config"
@@ -203,12 +204,20 @@ func Setup(
 			res := SendJoin(
 				httpReq, request, cfg, rsAPI, keys, roomID, eventID,
 			)
+			// not all responses get wrapped in [code, body]
+			var body interface{}
+			body = []interface{}{
+				res.Code, res.JSON,
+			}
+			jerr, ok := res.JSON.(*jsonerror.MatrixError)
+			if ok {
+				body = jerr
+			}
+
 			return util.JSONResponse{
 				Headers: res.Headers,
 				Code:    res.Code,
-				JSON: []interface{}{
-					res.Code, res.JSON,
-				},
+				JSON:    body,
 			}
 		},
 	)).Methods(http.MethodPut)