[Bug] Mongoist does not open new connection after close

[xfg]

I have created db.js module with code

//db.js
module.exports = mongoist('test');

Now I use db.js module within my tests in the following way

//foobar.spec.js
const db = require('./db');

describe('foo', () => {
  beforeEach(() => db.connect());
  afterEach(() => db.close());

  it('is some test', async () => {
    expect(await db.mycol.find({_id: 'foo'})).toBeNull();
  });

  it('is some other test', async () => {
    expect(await db.mycol.find({_id: 'bar'})).toBeNull();
  });
});

The second test will fall with an error MongoError: topology was destroyed because the connection will be closed after first test but not be opened anew before second test.

It is happen because Database#connect method always try to return the old connection even if it was be closed.

I think we should set this.connection to equal null within Database#close method about like this

return this.client
  .close(force)
  .then(() => this.connection = null)
  .catch((e) => {
  // Ignore this
    debug(`Could not close the connection due to error "${e.message}" ${e.stack}`);
  });