Fix user keying function returning null

src/store.test.ts

@@ -41,6 +41,7 @@ describe('Store with KeyingConfig', () => {
   it('generates keys from custom keying function', () => {
     const store = new Store(undefined, undefined, undefined, undefined, {
       User: () => 'me',
+      None: () => null,
     });
 
     expect(store.keyOfEntity({ __typename: 'Any', id: '123' })).toBe('Any:123');
@@ -49,6 +50,7 @@ describe('Store with KeyingConfig', () => {
     );
     expect(store.keyOfEntity({ __typename: 'Any' })).toBe(null);
     expect(store.keyOfEntity({ __typename: 'User' })).toBe('User:me');
+    expect(store.keyOfEntity({ __typename: 'None' })).toBe(null);
   });
 });
 

src/store.ts

@@ -157,7 +157,7 @@ export class Store {
 
     let key;
     if (this.keys[typename]) {
-      key = `${this.keys[typename](data)}`;
+      key = this.keys[typename](data);
     } else if (id !== undefined && id !== null) {
       key = `${id}`;
     } else if (_id !== undefined && _id !== null) {