fix: resolves issue proto-less object validation (#6884)

Due to `instanceof` comparison, object without prototype is mistaken for a privative value, throwing `TypeError: Cannot convert object to primitive value.` when trying to `.toString` said object (inside template string).

This mostly effects usage with `graphql` as `graphql-js`, [by design](graphql/graphql-js#504), creates objects by using Object.create(null).
This fix allows saving these objects.    

The fix uses `typeof` instead of `toString` comparison since I see no reason why `new Number/Boolean/Date` should be a supported use case.

Closes: #2065

src/persistence/EntityPersistExecutor.ts

@@ -41,7 +41,7 @@ export class EntityPersistExecutor {
     execute(): Promise<void> {
 
         // check if entity we are going to save is valid and is an object
-        if (!this.entity || !(this.entity instanceof Object))
+        if (!this.entity || typeof this.entity !== "object")
             return Promise.reject(new MustBeEntityError(this.mode, this.entity));
 
         // we MUST call "fake" resolve here to make sure all properties of lazily loaded relations are resolved

test/github-issues/2065/entity/Post.ts

@@ -0,0 +1,17 @@
+import { Entity, PrimaryColumn, Column } from "../../../../src";
+
+@Entity()
+export class Post {
+
+    @PrimaryColumn()
+    id: number;
+
+    @Column()
+    title: string;
+
+    constructor(id: number, title: string) {
+        this.id = id;
+        this.title = title;
+    }
+
+}

test/github-issues/2065/issue-2065.ts

@@ -0,0 +1,23 @@
+import "reflect-metadata";
+import {createTestingConnections, closeTestingConnections, reloadTestingDatabases} from "../../utils/test-utils";
+import {Connection} from "../../../src/connection/Connection";
+import {Post} from "./entity/Post";
+
+describe("github issues > #2065 TypeError: Cannot convert object to primitive value", () => {
+
+    let connections: Connection[];
+    before(async () => connections = await createTestingConnections({
+        entities: [__dirname + "/entity/*{.js,.ts}"],
+        schemaCreate: true,
+        dropSchema: true,
+    }));
+    beforeEach(() => reloadTestingDatabases(connections));
+    after(() => closeTestingConnections(connections));
+
+    it("should save an entity created with Object.create(null)", () => Promise.all(connections.map(async connection => {
+        const post = Object.create(null) as Post;
+        post.id = 1;
+        post.title = "Hello Post";
+        await connection.manager.save(Post, post);
+    })));
+});