Synopsys parser - optional value

Using the optional value setup `--foo[=bar]` still unsets the `--foo` while it shouldn't.

Noticed it when writing `wp core install` prompt.

This goes ok;

`wp core config --dbprefix="wp_" --dbname="zzpayer" --dbuser=root --dbpass`
Warning: --dbpass parameter needs a value
```Array
(
    [dbhost] => localhost
    [dbpass] => 
    [dbprefix] => wp_
    [locale] => nl_NL
    [dbname] => zzpayer
    [dbuser] => root
)
``` 

But that's because it gets merged with the defaults.

This not:

`wp core install --url --admin_email=mail@jai.me --admin_password=test --title=site ` results in 
```
Array
(
    [admin_email] => mail@jai.mel
    [admin_password] => test
    [title] => site
)
```
While they both have

`@synopsis --url[=<url>]`