Fix deserialization of missing parameters

Consider the following markup:

```html
      <form>
        <input type="checkbox" name="year" value="2012" checked>
        <input type="checkbox" name="year" value="2013" checked>
      </form>
```

deserializing these parameters `year=2012` should check '2012' and **uncheck** '2013'. The current implementation will check but not uncheck fields. This is true for all input types. This PR fixes the issue.