Document#becomes doesn't properly set the reference to @base in the errors object.

I ran into this problem when using Document#becomes to create a subclass and then validate it. Super simplified reproduction below. Just a rails 3.2.13 app with two models, one inheriting from the other.

``` ruby
class Klass
  include Mongoid::Document
end

class SubKlass < Klass
  field :attr, type: String
  validates :attr, presence: true
end
```

``` irb
Loading development environment (Rails 3.2.13)
1.9.3p392 :001 > k = Klass.new
 => #<Klass _id: 519aeff6be8a5cf65f000001, > 
1.9.3p392 :002 > k.errors.inspect
 => "#<ActiveModel::Errors:0x007ff5bf98a758 @base=#<Klass _id: 519aeff6be8a5cf65f000001, >, @messages={}>" 
1.9.3p392 :003 > sk = k.becomes SubKlass
 => #<SubKlass _id: 519aeff6be8a5cf65f000001, _type: "SubKlass", attr: nil> 
1.9.3p392 :004 > sk.errors.inspect
 => "#<ActiveModel::Errors:0x007ff5bf98a758 @base=#<Klass _id: 519aeff6be8a5cf65f000001, _type: nil>, @messages={}>" 
1.9.3p392 :005 > sk.valid?
NoMethodError: undefined method `attr' for #<Klass _id: 519aeff6be8a5cf65f000001, _type: nil>
1.9.3p392 :007 >   sk.instance_variable_set(:@errors, ActiveModel::Errors.new(sk))
 => #<ActiveModel::Errors:0x007ff5bf9df348 @base=#<SubKlass _id: 519aeff6be8a5cf65f000001, _type: "SubKlass", attr: nil>, @messages={}> 
1.9.3p392 :008 > sk.valid?
 => false 
```

I added one spec to test against the issue, and made the appropriate changes to Document#becomes.