Flip ordering of path/shard args to Call

To match the previous calling convention for DoJSONHTTPRequest and
reduce confusion when migrating from the old API.

clientutil/backend_test.go

@@ -161,7 +161,7 @@ func doJSONRequests(backends *testBackends, u string, n int, shards []string) (i
 		if len(shards) > 0 {
 			shard = shards[rand.Intn(len(shards))]
 		}
-		err = b.Call(ctx, "/", shard, struct{}{}, &resp)
+		err = b.Call(ctx, shard, "/", struct{}{}, &resp)
 		cancel()
 		if err != nil {
 			errs++

clientutil/balancer.go

@@ -97,7 +97,7 @@ func newBalancedBackend(config *BackendConfig, resolver resolver) (*balancedBack
 // Call the backend. Makes an HTTP POST request to the specified uri,
 // with a JSON-encoded request body. It will attempt to decode the
 // response body as JSON.
-func (b *balancedBackend) Call(ctx context.Context, path, shard string, req, resp interface{}) error {
+func (b *balancedBackend) Call(ctx context.Context, shard, path string, req, resp interface{}) error {
 	data, err := json.Marshal(req)
 	if err != nil {
 		return err