Fix `update --all`

After 6dec5cb7aea69dc4aa0e7586f2c16732aefab6b0, `wp plugin update --all` doesn't work anymore, because `<plugin>...` is not optional in its synopsis.

Originally reported by @danielbachhuber in https://github.com/wp-cli/wp-cli/pull/720#issuecomment-24178700

The bigger problem is that we don't have any tests for `update --all`.