Add `ObjectMapper.updateValue()` method to update instance with given overrides

ObjectMapper already have method to convertValue by doing two-step conversion from given value, into instance of given value type.

It will be great if ObjectMapper can update already created instance.

public void convertValue(Object fromValue, Object toValue) 
               throws IllegalArgumentException {
    if (fromValue == null || toValue == null) {
        return;
    }
    JavaType toValueType = _typeFactory.constructType(toValue.getClass());
    TokenBuffer buf = new TokenBuffer(this, false);
    if (isEnabled(DeserializationFeature.USE_BIG_DECIMAL_FOR_FLOATS)) {
        buf = buf.forceUseOfBigDecimal(true);
    }
    try {
        SerializationConfig config = getSerializationConfig().
                without(SerializationFeature.WRAP_ROOT_VALUE);
        _serializerProvider(config).serializeValue(buf, fromValue);
        JsonParser jp = buf.asParser();
        DeserializationConfig deserConfig = getDeserializationConfig();
        JsonToken t = _initForReading(jp);
        if (t != JsonToken.VALUE_NULL
                && t != JsonToken.END_ARRAY
                && t != JsonToken.END_OBJECT) {
            DeserializationContext ctxt = createDeserializationContext(jp, deserConfig);
            JsonDeserializer<Object> deser = _findRootDeserializer(ctxt, toValueType);
            deser.deserialize(jp, ctxt, toValue);
        }
        jp.close();
    } catch (IOException e) { // should not occur, no real i/o...
        throw new IllegalArgumentException(e.getMessage(), e);
    }
}

[cowtowncoder]

Decided to make semantics such that whatever the result of logical merge is is returned: this is usually first argument, but not always – Java arrays, for example, can not be changed (with respect to length), so updateValue() on an array will succeed, but has to create a new instance and return that. But type should be the same so usage should be convenient and allow chaining.

Testing so far is limited to shallow updates, but will try to find time for deep merge: this will, however, only work via #1399 that is, configuration and/or annotations need to indicate the need for merging.

[cowtowncoder]

@cvc-saksoft Usage questions should generally be sent to mailing list (https://groups.google.com/forum/#!forum/jackson-user) or Gitter (https://gitter.im/FasterXML/jackson-databind).

But as to naming: property names need to match, so incoming JSON just has to map to type, same as if just using readValue(). No special handling is applied aside from skipping construction of value. Type parameter is used for finding deserializer to use.