npm update --save mixes devDependencies into dependencies in package.json

When you run `npm-update --save|--save-dev|--save-optional`, all packages that needed updating will magically appear in the section specified by `--save|--save-dev|--save-optional`,  regardless of whether they were originally regular, development or optional dependencies. :-1: .

Example of running `npm update --save` on a simple `package.json` with only a development dependency:

#### Before
```json
{
  "name": "update-save-test",
  "version": "0.5.0",
  "devDependencies": {
    "rimraf": "~2.0.0"
  }
}

```
#### After
```json
{
  "name": "update-save-test",
  "version": "0.5.0",
  "devDependencies": {
    "rimraf": "~2.0.0"
  },
  "dependencies": {
    "rimraf": "~2.0.3"
  }
}

```