Added DefaultMapper support to map only partial fields from result instead of whole source field.

ES was properly queried for only part of the fields but DefaultMapper did not support mapping of partial result - simply returned null as result object - this change introduces mapping of partial fieldset to desired class.
Usage:
Consider example class:
```java
public class Person{
   private String name;
   private String surname;
   private Integer age;
}
```
now one can query only selected fields using:
```java
NativeSearchQueryBuilder builder = new NativeSearchQueryBuilder()
                .withTypes("person")
                .withFields("name", "age");
```
and it will actually work.