Toggle primary email visibility

## Endpoint
`PATCH /user/email/visibility`

## Expected response
`200 OK`

### Docs
[Toggle primary email visibility](https://developer.github.com/v3/users/emails/#toggle-primary-email-visibility)

### Example JSON Response
```json
[
  {
    "email": "octocat@github.com",
    "primary": true,
    "verified": true,
    "visibility": "private"
  }
]
```