Description/Steps to reproduce

When instantiating a Connector with settings including url and some other setting, the other settings are ignored.

Example of a test which wouldn't pass:

it('honours multiple user-defined settings', function() {
  var urlOnly = {url: newConfig(true).url, max: 999};
  var dataSource = new DataSource(connector, urlOnly);
  var pool = dataSource.connector.pg;
  
  pool.options.max.should.equal(999);
  var clientConfig = dataSource.connector.clientConfig;
  clientConfig.connectionString.should.equal(urlOnly.url);
});

Expected result

In this test, the max value in the pool should be 999 but it is at its default value of 10.

Additional information

I found where the problem come from:

https://github.com/strongloop/loopback-connector-postgresql/blob/eca1bb8fce8045e6a96d816ae218804fe51f8b6f/lib/postgresql.js#L75-L80

In the case of an url, the entire clientConfig is replaced by an object with only the URL key

I think this is a problem because in 3.x, this case is handled and included in tests.