Incorrect mocking of `req.query`.

[AlexChalk]

If I use superagent and set query to { foo: undefined, bar: [], baz: false, quux: ['a', 'b'] }, superagent-mock generates a req.query object of { foo: 'undefined', bar: '', baz: 'false', quux: 'a,b' }. However, in reality my api receives a query object of { baz: 'false', quux: ['a', 'b'] }.

Steps to reproduce:

1. Write mock for a superagent request:

   mock.get(...api-address..., (req) => {
   console.log(req.query);
   return { status: 200 };
   });

2. Write request and verify that { foo: 'undefined', bar: '', baz: 'false', quux: 'a,b' } is logged to console:

   request
   .get(`...api-address...`)
   .query({ foo: undefined, bar: [], baz: false, quux: ['a', 'b'] })

3. Send the request to your server and log req.query, confirm it equals { baz: 'false', quux: ['a', 'b'] }.