Client: always call ensureReaderClosed

Unlike a plain `net/http/client.Do()`, requests made through client/request
use the `sendRequest` function, which parses the server response, and may
convert non-transport errors into errors (through `cli.checkResponseErr()`).

This means that we cannot assume that no reader was opened if an error is
returned.

This patch changes various locations where `ensureReaderClosed` was only
called in the non-error situation, and uses a `defer` to make sure it's
always called.

`ensureReaderClosed` itself already checks if the response's body was set,
so in situations where the error was due to a transport error, calling
`ensureReaderClosed` should be a no-op.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

client/container_diff.go

@@ -13,11 +13,11 @@ func (cli *Client) ContainerDiff(ctx context.Context, containerID string) ([]con
 	var changes []container.ContainerChangeResponseItem
 
 	serverResp, err := cli.get(ctx, "/containers/"+containerID+"/changes", url.Values{}, nil)
+	defer ensureReaderClosed(serverResp)
 	if err != nil {
 		return changes, err
 	}
 
 	err = json.NewDecoder(serverResp.body).Decode(&changes)
-	ensureReaderClosed(serverResp)
 	return changes, err
 }