title: Update a undefined proprety on mongoose schema and save
body: <p>I have defined a user schema like below</p>
  
  <pre><code>var UserSchema = new Schema({
    firstName: { type: String, required: true },
    lastName: { type: String, required: true },
    email: { type: String, required: true },
    location: String,
    picture: String,
    isFirstNamePublic: { type: Boolean, required: false, Default: false },
    isLastNamePublic: { type: Boolean, required: false, Default: false },
    isEmailPublic: { type: Boolean, required: false, Default: false },
    isLocationPublic: { type: Boolean, required: false, Default: false },
    isPicturePublic: { type: Boolean, required: false, Default: false },
    passwordHash: { type: String, required: true },
    userRoles: [{businessId: String, role: {type: String, uppercase: true, enum: ['INDIVIDUAL_USER', 'BUSINESS_EMPLOYEE', 'BUSINESS_ADMIN', 'CLIENT', 'PLATFORM_EMPLOYEE']}}],
    interestedOfferCategory: String,
    linkedSocialNetwork: {type: String, required: false, uppercase: true, enum: ['FACEBOOK', 'TWITTER', 'LINKEDIN']},
    linkedSocialNetworkUserId: String,
    resetPasswordToken: String,
    resetPasswordExpired: Boolean
  });
  </code></pre>
  
  <p>Now If I want to update any field that is undefined I am not able to update. For ex: I have a user document saved like below</p>
  
  <pre><code>{ _id: 54c439395ff4ede519912ea8,
    firstName: 'updated first name',
    lastName: 'test',
    email: 'test@test.com',
    interestedOfferCategory: 'testcategory',
    passwordHash: '$2a$10$S1zqmQHQ7Sd/qJjtryfzFua4Rn6Xi/oCHsJP9mLfyUnMg/gNV/u3K',
    __v: 0,
    userRoles: 
     [ { businessId: '123',
         role: 'INDIVIDUAL_USER',
         _id: 54c439395ff4ede519912ea9 } ] }
  </code></pre>
  
  <p>Now I extend this document to add resetPasswordToken like below</p>
  
  <pre><code>var updatedUser = _.extend(user, {resetPasswordToken: token, revokeSessionTokenExpired: false});
  </code></pre>
  
  <p>And If I try to save the document the values of resetPasswordToken and resetPasswordTokenExpired is not updated.</p>
  
  <pre><code>updatedUser.save(cb);
  </code></pre>
  
  <p>The above line not updating values.</p>
  
  <p>What am I missing here?</p>