title: Find by ObjectID using mongoose always returns null
body: <p>I want to search a document with _id property which is of ObjectId type. But it keeps returning null even though a document with exact _id exists.</p>
  
  <pre><code>const employee = require("../Models/employee");
  
  router.get('/employees/:id', function(req,res){
      console.log(req.params.id);
      employee.findById(req.params.id).exec(function(err, emp){
          if(err) {
              res.send("Error! " + err);
          } else {
              res.json(emp);
          }
      })
  });
  </code></pre>
  
  <p>employee model:</p>
  
  <pre><code>const mongoose = require("mongoose");
  
  const Schema = mongoose.Schema;
  
  const empSchema = new Schema({
      _id: String,
      Name: String,
      DOB: String,
      Sex: String,
      Experience: Number,
      Expertise: String,
  });
  
  module.exports = mongoose.model('employee', empSchema, 'EmpDetails');
  </code></pre>
  
  <p>MongoDB has the _id:</p>
  
  <pre><code>{ 
      "_id" : ObjectId("59eebab187179c880ed9473b"), 
      "Name" : "Abhisek", 
      "DOB" : "04/28/1991", 
      "Sex" : "Male", 
      "Experience" : 4.0, 
      "Expertise" : "Javascript"
  }
  </code></pre>
  
  <p>Console also displays the same</p>
  
  <pre><code>59eebab187179c880ed9473b
  </code></pre>