Bug: `wp option update blog_public` produces error

See behavior exhibited here:

```
hmn-vagrant ➜  wordpress-trunk.dev:  wp option update blog_public 0
Success: Updated 'blog_public' option.
hmn-vagrant ➜  wordpress-trunk.dev:  wp option update blog_public 0
Error: Could not update option 'blog_public'.
hmn-vagrant ➜  wordpress-trunk.dev:  wp option update blog_public 1
Success: Updated 'blog_public' option.
hmn-vagrant ➜  wordpress-trunk.dev:  wp option update blog_public 1
Error: Could not update option 'blog_public'.
```

Essentially, if the option is already set to the value, an error is thrown.

Options with non-numeric values don't seem to be affected.

Originally reported by @nikolay