leftJoinAndMapMany is not mapping the result to the property when I pass a Subquery to the second argument

Issue type: [X] bug

Database system/driver: [X] postgres

TypeORM version: [X] latest

Steps to reproduce:

1. Clone this repo
2. npm install && tsc
3. npm start

These are the entities:

@Entity()
export class Category {
    @PrimaryGeneratedColumn()
    id: number;
    @Column()
    name: string;

    posts: Post[];
}

@Entity()
export class Post {
    @PrimaryGeneratedColumn()
    id: number;
    @Column()
    title: string;
    @Column('int')
    categoryId: number;
}

And this is how I build the queries:

This one returns the list of Posts:

connection.createQueryBuilder().select('category').from(Category, 'category')
              .leftJoinAndMapMany('category.posts', Post, 'post', 'post.categoryId = category.id');

This one does not:

connection.createQueryBuilder().select('category').from(Category, 'category')
                    .leftJoinAndMapMany('category.posts', qb => qb.select().from(Post, 'post'), 'post', 'post."categoryId" = category.id');