Add `MapperFeature.WRAPPERS_DEFAULT_TO_NULL` to allow configuring `JsonInclude.Include.NON_DEFAULT` default for primitive wrapper types to be `null`, not wrapped default of primitive

[cowtowncoder]

(note: follow-up on #2105)

So: it looks like NON_DEFAULT value for Boolean -- for example -- is new Boolean(false) (similar to the default of false for primitive boolean). But this does not really make sense: for non-primitive types, default should be null.

NOTE: this is different from Class-level "Class member Default" which compares actual property values a POJO is initialized with, using default (no-args) Constructor.

Test case:

public class Test {

@Test
public void name() throws Exception {
  ObjectMapper mapper = JsonMapper.builder()
    .changeDefaultPropertyInclusion(incl -> incl.withValueInclusion(JsonInclude.Include.NON_DEFAULT))
    .build();
  Pojo pojo = new Pojo();
  pojo.setValue(false);

  System.out.println(mapper.writeValueAsString(pojo));
}

public static class Pojo {
  private Boolean value;

  public Boolean getValue() {
    return value;
  }

  public void setValue(Boolean value) {
    this.value = value;
  }
}

with Jackson v 2.8.2 outputs {"value":false} (as I expected). This however changes starting with 2.8.3: it outputs {} (tested with 2.8.3, 2.8.11.2 and 2.9.6).
Default value for Boolean is null, not false.

EDIT: behavior will be controlled with a new MapperFeature.WRAPPERS_DEFAULT_TO_NULL -- planned to default to false in 3.1 (leaving existing 2.8.3 - 3.0.3 behavior as default).

[cowtowncoder]

Thinking about this bit more, I think we need a MapperFeature to allow toggling this, for backwards compatibility.