--save-exact does not work.

With commitizen@2.8.6, It doesn't make any difference:

```
$ commitizen init cz-conventional-changelog --save-dev --save-exact
$ cat package.json
{
"devDependencies": {
"cz-conventional-changelog": "^1.2.0"
},
"config": {
"commitizen": {
"path": "./node_modules/cz-conventional-changelog"
}
}
}
```

```
$ commitizen init cz-conventional-changelog --save-dev
$ cat package.json
{
"devDependencies": {
"cz-conventional-changelog": "^1.2.0"
},
"config": {
"commitizen": {
"path": "./node_modules/cz-conventional-changelog"
}
}
}
```

Reproduce the two commitizen init commands shown in the issue and compare the resulting package.json dependency version. Trace the init command's handling of --save-exact and the package metadata update; done means the flag produces an exact version instead of a caret range, while omitting it retains the current behavior.

Vom Indexierungsmodell aus dem Issue-Text verfasst.