Fix spec failures for Mongoid 3.

This has no actual changes to Devise itself, just fixes the failing
tests when running against Mongoid 3 instead of Mongoid 2.

Mocha has been locked at 0.10.0 since 0.12.0 raises an error when trying
to set an expectation on a frozen object.

Tests were updated to work with both AR and Mongoid, some cases the XML
serialization was slightly different but both were outputting correct
and valid XML, and the id/_id field mismatch is now handled.

An active field was missing from the test models for Mongoid, and the
invalid :null => true options in field were removed.

test/orm/mongoid.rb

@@ -1,14 +1,13 @@
 require 'mongoid/version'
 
 Mongoid.configure do |config|
-  config.master  = Mongo::Connection.new('127.0.0.1', 27017).db("devise-test-suite")
+  config.connect_to("devise-test-suite")
   config.use_utc = true
   config.include_root_in_json = true
 end
 
 class ActiveSupport::TestCase
   setup do
-    User.delete_all
-    Admin.delete_all
+    Mongoid.purge!
   end
 end