message: Add unknown serializer
diff: import { deepMerge } from '@orbit/utils';
  import {
  + NoopSerializer,
  BooleanSerializer,
  StringSerializer,
  DateSerializer,
  @@ -30,6 +31,7 @@ export function buildJSONAPISerializerFor(settings: {
  const { schema, keyMap } = settings;
  const defaultSerializerClassFor = buildSerializerClassFor({
  + unknown: NoopSerializer,
  boolean: BooleanSerializer,
  string: StringSerializer,
  date: DateSerializer,
  @@ -47,7 +49,7 @@ export function buildJSONAPISerializerFor(settings: {
  });
  let serializerClassFor: SerializerClassForFn;
  if (settings.serializerClassFor) {
  - serializerClassFor = (type: string) => {
  + serializerClassFor = (type = 'unknown') => {
  return (
  settings.serializerClassFor(type) || defaultSerializerClassFor(type)
  );
  @@ -73,7 +75,7 @@ export function buildJSONAPISerializerFor(settings: {
  });
  let customSerializerSettingsFor = settings.serializerSettingsFor;
  if (customSerializerSettingsFor) {
  - serializerSettingsFor = (type: string) => {
  + serializerSettingsFor = (type = 'unknown') => {
  let defaultSerializerSettings = defaultSerializerSettingsFor(type) || {};
  let customSerializerSettings = customSerializerSettingsFor(type) || {};
  return deepMerge(defaultSerializerSettings, customSerializerSettings);
  @@ -88,9 +90,9 @@ export function buildJSONAPISerializerFor(settings: {
  serializerSettingsFor
  });
  if (customSerializerFor) {
  - return (type: string) =>
  + return (type = 'unknown') =>
  customSerializerFor(type) || backupSerializerFor(type);
  } else {
  - return (type: string) => backupSerializerFor(type);
  + return (type = 'unknown') => backupSerializerFor(type);
  }
  }
  