title: Can't call devise methods as soon as I upgraded to Mongoid 5
body: <p>Below is the call trace...</p>
  
  <pre><code>NoMethodError - undefined method `each_pair' for :primary_preferred:Symbol:
    bson (3.2.4) lib/bson/document.rb:82:in `initialize'
    mongo (2.1.1) lib/mongo/client.rb:192:in `read_preference'
    /Users/Sagar/.rvm/rubies/ruby-2.2.1/lib/ruby/2.2.0/forwardable.rb:183:in `read_preference'
    mongo (2.1.1) lib/mongo/collection.rb:104:in `read_preference'
    /Users/Sagar/.rvm/rubies/ruby-2.2.1/lib/ruby/2.2.0/forwardable.rb:183:in `read_preference'
    mongo (2.1.1) lib/mongo/collection/view/readable.rb:411:in `default_read'
    mongo (2.1.1) lib/mongo/collection/view/readable.rb:300:in `read'
    mongo (2.1.1) lib/mongo/collection/view/iterable.rb:40:in `block in each'
    mongo (2.1.1) lib/mongo/retryable.rb:46:in `read_with_retry'
    mongo (2.1.1) lib/mongo/collection/view/iterable.rb:39:in `each'
    mongoid (5.0.0) lib/mongoid/query_cache.rb:207:in `each'
    mongoid (5.0.0) lib/mongoid/contextual/mongo.rb:240:in `block in first'
    mongoid (5.0.0) lib/mongoid/contextual/mongo.rb:489:in `try_cache'
    mongoid (5.0.0) lib/mongoid/contextual/mongo.rb:239:in `first'
    mongoid (5.0.0) lib/mongoid/contextual.rb:20:in `first'
    orm_adapter (0.5.0) lib/orm_adapter/adapters/mongoid.rb:22:in `get'
    devise (3.4.1) lib/devise/models/authenticatable.rb:214:in `serialize_from_session'
    devise (3.4.1) lib/devise.rb:467:in `block (2 levels) in configure_warden!'
    warden (1.2.3) lib/warden/session_serializer.rb:34:in `fetch'
    warden (1.2.3) lib/warden/proxy.rb:212:in `user'
    warden (1.2.3) lib/warden/proxy.rb:318:in `_perform_authentication'
    warden (1.2.3) lib/warden/proxy.rb:104:in `authenticate'
    devise (3.4.1) lib/devise/controllers/helpers.rb:120:in `current_user'
    devise (3.4.1) lib/devise/controllers/helpers.rb:116:in `user_signed_in?'
  </code></pre>
  
  <p>Everything was working perfectly fine with Mongoid 4.2 and as soon as upgraded Mongoid to 5.0.0 I found this one!</p>
  
  <p>Could you please help me out with this one? Not a clue how to debug this.
  Thank you in advance.</p>