Fix convert $in operators with single value to array

[talesdsp]

As an example

import { middleware as query } from 'querymen';

app.get('/posts', query({
  categories: {
    type: [String],
    operator: '$in'
  }
}));

// User requests /posts?categories=test querymen before would return:
// { categories: { $in: 'test' } }

// Now:
// { categories: { $in: ['test'] } }