Json Merge set null (remove) value for merge pathes (following #1399 and #1437)

[Tetradeus]

Version : 2.9 r2

Related to #1399 and #1437

Wouldn't it be nice to have an option to enable setting null values at merge pathes ? Ref #1437

Presently in tests about Json merge :

    // Also: nulls better not override
    public void testBeanMergingWithNull() throws Exception
    {
        Config config = MAPPER.readerForUpdating(new Config(5, 7))
                .readValue(aposToQuotes("{'loc':null}"));
        assertNotNull(config.loc);
        assertEquals(5, config.loc.a);
        assertEquals(7, config.loc.b);
    }

What if I really want to be able to set null for loc ? assertNull(config.loc);
Not totally sure, but this would make it Json Merge Patch compatible.

[cowtowncoder]

Sounds reasonable. May need to re-wire handling of @JsonMerge quite significantly but other than that.

[cowtowncoder]

Toying with an alternate idea of combining this with @JsonSetter(nulls=Nulls.SET) (vs Nulls.SKIP).
On one hand, that setting already exists, and would work well for per-property annotations.
But on the other hand might be bit messy for defaulting (per-type, globals); and needs of merging may be different from use for straight "regular" deserialization.

[cowtowncoder]

Actually going with @JsonSetter route makes tons of sense because that is fully wired already, and conversely trying to use different settings for merge/non-merge case would be a major PITA.

So: changes I will make are:

1. Default for merge for nulls is same as regular deserialization setting: that is, just set it as null (or replacement null value as case may be)
2. To skip nulls during merging, JsonSetter.Value must be used at one of 3 levels (property annotation, per value-type annotation, global defaults) -- Nulls.SKIP will achieve this.

And with that it will be possible to make nulls overwrite non-null values.