fix: To one relationships, that have a nested destination keypath are not correctly mapped to JSON

I have two NSManagedObjects A and B. A has a "to one" relationship to B named "b".
In the JSON there is an additional nesting object named "header".

The JSON result should look like:
```json
{
    "a":{
        "header":{
            "b": "<b-value>",
            /*...*/
        },
        /*...*/
}
```

But the result looks like:

```json
{
    "a":{
        "header.b": "<b-value>"
    },
    /*...*/
}
```

It looks like the problem was in RKMappingOperation.m.
The method "mapOneToOneRelationshipWithValue:mapping:" sets the value in "destinationObject" with "setValue:forKey:" instead of "setValue:forKeyPath:".
In contrast the method "mapOneToManyRelationshipWithValue:mapping:" uses "setValue:forKeyPath:".

I'm not 100% sure, if there's a reason, that the "to one" method uses "setValue:forKey:" instead of "setValue:forKeyPath:", but i think this is a bug.