Allow both :_source and :_fields rather than either/or

When using the ```:fields``` option when searching will only return the values of those fields if ```_source``` is not one of the fields.  If ```_source``` is one of the fields then no other fields will be returned for a search.

There are, however, situations when this may be useful.  For example, a search that is needed to return the document itself and the value of ```_parent``` field.

The change in this pull request will always return the fields if any are specified in addition to ```_source```.