Sinon: Default sandbox's restore method does not restore stubs/spies

When using any sandbox method on the default sandbox, restore() does not restore mocks or spies properly. A minimal repro case on Sinon v5.0.2:

const sinon = require('sinon');
const myApi = {
  someMethod() {
    console.log('test method!');
  }
};

sinon.stub(myApi, 'someMethod');
sinon.restore();
sinon.stub(myApi, 'someMethod');
// TypeError: Attempted to wrap someMethod which is already wrapped

It seems that in the call to extend in lib/sinon.js, the un-sandboxed stub/spy/etc. exports from apiMethods end up shadowing the sandbox methods, and so restore() doesn't do anything. The Sandbox API docs and source in lib/sinon.js seem to suggest that this is intended usage.

I only tested stub and spy, but I would expect any other shadowed methods also do not work as intended.

I ran these tests on Node v8.9.4 and Chrome 66.0.3359.117 via Karma/Chromedriver, though this issue seems unrelated to environment.