title: Unable to create document in mongoid with field type array
body: <p>Following is my model which has a field with type array</p>
  
  <pre><code>class User::Log::Presentation
    include Mongoid::Document
    include Mongoid::Timestamps
  
    field :name, type: String
    field :fields, type: Array, default: []
  end
  </code></pre>
  
  <p>When I try creating a document, it throws following error.</p>
  
  <pre><code>TypeError: no implicit conversion of String into Integer
  </code></pre>
  
  <p>I am totally lost with this. Couldnt find any answer is other questions. Please help.</p>