Key type of `Map` valued properties not being added to Schema

Looks like `MapVisitor.java` is not doing anything with key type it gets via callback:

``` java
@Override
public void keyFormat(JsonFormatVisitable handler, JavaType keyType)
throws JsonMappingException {
// JSON Schema only allows String types so let's not bother too much
}
```

but it should, since there is some useful information from databinding perspective (enum, time/date types, numbers-as-Strings) and this should be available from `JsonSchema` object.