title: Conditional query in Mongoose where params may be missing
body: <p>I want to pass through values for a search using Mongoose. This doesn't work... I don't think...
  <a href="https://stackoverflow.com/questions/19693029/how-to-build-a-conditional-query-in-mongoose">How to build a conditional query in Mongoose?</a>
  As I will want to apply logic to the value, for example with dates I will pass through two values and do greater than and less than. This is purely an = query.</p>