title: Mongoose update without callback
body: <p>I have a typical schema and model:</p>
  
  <pre><code>var mongoose = require('mongoose');
  
  var userSchema = new mongoose.Schema({
      email: String,
      password: String,
      profile: {
          name: String,
          surname: String,
          photo: String
      },
      stats: {
          lastLogin: { type: Date, default: Date.now },
          loginCount: Number,
          lastIP: String
      },
      source: String,
      deleted: Boolean,
      dateCreated: { type: Date, default: Date.now }
  });
  
  mongoose.model('User', userSchema);
  </code></pre>
  
  <p>When I perform this update, it only works if I define the callback, else it simply executes but no value is changed in the database:</p>
  
  <pre><code>User.update({email:'foo@bar.com'}, {$inc: {'stats.loginCount': 1}});
  </code></pre>
  
  <p>This works:</p>
  
  <pre><code>User.update({email:'foo@bar.com'}, {$inc: {'stats.loginCount': 1}}, function() {});
  </code></pre>
  
  <p>Is this a bug? I don't see in the documentation if the callback is required but it's strange to require this… I think i'm missing something here.</p>
  
  <p>Notes: I'm matching by email for testing proposes, I'm using mongoose v3.5.4 in NodeJS v0.8.17 with a simple Express v3.0.6 setup.</p>
  
  <p>Thanks in advance.</p>