Fix mapped name behavior when using serializer default exclusion.

During serialization, when using `only` in combination with  `nestedAttribute`, the behavior seems a little strange.

I would expect 

```javascript
railsSerializer(function() {
  this.nestedAttribute('foo');
  this.only('foo');
});
```

to serialize `foo` as `foo_attributes`. However it does not get serialize. Neither it does when using `this.only('foo_attributes');` instead. 
After looking a little around the source code, I found at that 

```javascript
railsSerializer(function() {
  this.nestedAttribute('foo');
  this.only('foo', 'foo_attributes');
});
```

would give the wanted result. However, I do not think this is how we really want `only` to behave.

I fixed the serialization process so that when using default exclusion, if either the attribute name or the mapped name is accepted, the attribute get serialized. I also provided a basic unit test for the above case.
This should not change anything about the behavior when `only` is not used.