datasource: copy settings object in constructor

Fix DataSource constructor to create a shallow copy of the settings
object provided by the caller. This prevents surprising behavior
where changes made to `ds.settings` were picked up by the provided
config object, as observed e.g. in tests of our MongoDB connector.

lib/datasource.js

@@ -126,6 +126,12 @@ function DataSource(name, settings, modelBuilder) {
     settings = utils.parseSettings(settings);
   }
 
+  // Shallow-clone the settings so that updates to `ds.settings`
+  // do not modify the original object provided via constructor arguments
+  if (settings) settings = Object.assign({}, settings);
+  // It's possible to provide settings object via the "name" arg too!
+  if (typeof name === 'object') name = Object.assign({}, name);
+
   this.modelBuilder = modelBuilder || new ModelBuilder();
   this.models = this.modelBuilder.models;
   this.definitions = this.modelBuilder.definitions;

test/datasource.test.js

@@ -9,6 +9,15 @@ const should = require('./init.js');
 const DataSource = require('../lib/datasource.js').DataSource;
 
 describe('DataSource', function() {
+  it('clones settings to prevent surprising changes in passed args', () => {
+    const config = {connector: 'memory'};
+
+    const ds = new DataSource(config);
+    ds.settings.extra = true;
+
+    config.should.eql({connector: 'memory'});
+  });
+
   it('reports helpful error when connector init throws', function() {
     const throwingConnector = {
       name: 'loopback-connector-throwing',