title: json post to devise causes 422 / Unprocessable entity
body: <p>i'm using angular and posting to a RoR back-end, with devise for auth. When I submit a new user for creation, I'm getting the following:</p>
  
  <pre><code>Started PUT "/api/user" for 127.0.0.1 at 2013-11-19 11:38:13 -0600
  Processing by Devise::RegistrationsController#create as JSON
    Parameters: {"user"=&gt;{"email"=&gt;"alex@someemail.com", "password"=&gt;"[FILTERED]", "password_confirmation"=&gt;"[FILTERED]", "first_name"=&gt;"alex", "last_name"=&gt;"p"}, "registration"=&gt;{"user"=&gt;{"email"=&gt;"alex@someemail.com", "password"=&gt;"[FILTERED]", "password_confirmation"=&gt;"[FILTERED]", "first_name"=&gt;"alex", "last_name"=&gt;"p"}}}
     (0.2ms)  BEGIN
    User Exists (0.8ms)  SELECT 1 AS one FROM "users" WHERE "users"."email" = 'alex@someemail.com' LIMIT 1
     (0.2ms)  ROLLBACK
  Completed 422 Unprocessable Entity in 128ms (Views: 0.2ms | ActiveRecord: 7.4ms)
  </code></pre>
  
  <p>User model:</p>
  
  <pre><code>class User &lt; ActiveRecord::Base
    # Include default devise modules. Others available are:
    # :token_authenticatable, :confirmable,
    # :lockable, :timeoutable and :omniauthable
    devise :database_authenticatable, :registerable,
           :recoverable, :rememberable, :trackable, :validatable
    attr_accessible :first_name, :last_name, :email, :password, :password_confirmation, :remember_me, :last_check
  
  end
  </code></pre>
  
  <p>I can't figure out what's going on. I assume it's something with the parameters I'm sending (or not sending?) but everything seems to be there.</p>