Add customization option for LocalDateTime handling of seconds and nanos

Hi all,

as seen in https://github.com/FasterXML/jackson-modules-java8/blob/master/datetime/src/main/java/com/fasterxml/jackson/datatype/jsr310/ser/LocalDateTimeSerializer.java method _serializeAsArrayContents line 112, if seconds or nanos are set to 0 in the LocalDateTime object being serialized, they will NOT be included into the resulting JSON.

I understand this might be trying to follow what its toString https://docs.oracle.com/javase/8/docs/api/java/time/LocalDateTime.html#toString does, but it is not convenient when senders and receivers either do not understand the LocalDateTime specification or are unable to handle it properly.

Please add a customization option to allow sending seconds and nanos even when set to 0, the only current workaround is to implement a custom serializer and add it to a custom mapper to be passed in whatever configuration needs it, which is not really convenient:

````
public static class LocalDateTimeWithSecondsSerializer extends StdSerializer {

public LocalDateTimeWithSecondsSerializer() {
super(LocalDateTime.class);
}

@Override
public void serialize(LocalDateTime value, JsonGenerator gen, SerializerProvider provider) throws IOException {
gen.writeStartArray();
gen.writeNumber(value.getYear());
gen.writeNumber(value.getMonthValue());
gen.writeNumber(value.getDayOfMonth());
gen.writeNumber(value.getHour());
gen.writeNumber(value.getMinute());
gen.writeNumber(value.getSecond());
gen.writeEndArray();
}
}

public static class CustomEntityMapper implements EntityMapper {

private final ObjectMapper objectMapper;

public CustomEntityMapper() {

SimpleModule localDateTimeWithSecondsSerializerModule = new SimpleModule();
localDateTimeWithSecondsSerializerModule.addSerializer(new LocalDateTimeWithSecondsSerializer());

objectMapper = new ObjectMapper()
.registerModule(new JavaTimeModule())
.registerModule(localDateTimeWithSecondsSerializerModule)
.registerModule(new Jdk8Module());

objectMapper.findAndRegisterModules();
}

@Override
public String mapToString(Object object) throws IOException {
return objectMapper.writeValueAsString(object);
}

@Override
public T mapToObject(String source, Class clazz) throws IOException {
return objectMapper.readValue(source, clazz);
}
}
````

Thanks and cheers