Fix uniqueness matcher when scope is a *_type attr

This is a fix for the long-standing #203, and is taken from #366.

----

This commit fixes `validate_uniqueness_of` when used with `scoped_to` so
that when one of the scope attributes is a polymorphic *_type attribute
and the model has another validation on the same attribute, the matcher
does not fail with an error.

As a part of the matching process, `validate_uniqueness_of` tries to
create two records that have the same values for each of the attributes
passed to `scoped_to`, except one of the attributes has a different
value. This way, the second record should be valid because it shouldn't
clash with the first one. It does this one attribute at a time.

Let's say the attribute in question is a polymorphic *_type attribute.
The value of this attribute is intended to be the name of a model as a
string. Let's assume the first record has a meaningful value for this
attribute already and we're trying to find a value for the second
record. In order to produce such a value, `validate_uniqueness_of`
generally calls #next (a common method in Ruby to generate a succeeding
version of an object sequentially) on the first object's corresponding
value. So in the case of a *_type attribute, since it's a string, it
would call #next on that string. For instance, "User" would become
"Uses".

You might have noticed a problem with this, which is "what if Uses is
not a valid model?" This is okay as long as there's nothing that is
trying to access the polymorphic association. Because as soon as this
happens, Rails will attempt to find a record using the polymorphic type
-- in other words, it will try to find the model that the *_type
attribute corresponds to. One of the ways this can happen is if the
*_type attribute in question has a validation on it itself.

Let's look at an example.

Given these models:

``` ruby
class User < ActiveRecord::Base
end

class Favorite < ActiveRecord::Base
  belongs_to :favoriteable, polymorphic: true
  validates :favoriteable, presence: true
  validates :favoriteable_id, uniqueness: { scope: [:favoriteable_type] }
end

FactoryGirl.define do
  factory :user

  factory :favorite do
    association :favoriteable, factory: :user
  end
end
```

and the following test:

``` ruby
require 'rails_helper'

describe Favorite do
  context 'validations' do
    before { FactoryGirl.create(:favorite) }
    it do
      should validate_uniqueness_of(:favoriteable_id).
        scoped_to(:favoriteable_type)
    end
  end
end
```

prior to this commit, the test would have failed with:

```
Failures:

  1) Favorite validations should require case sensitive unique value for favoriteable_id scoped to favoriteable_type
     Failure/Error: should validate_uniqueness_of(:favoriteable_id).
     NameError:
       uninitialized constant Uses
     # ./spec/models/favorite_spec.rb:6:in `block (3 levels) in <top (required)>'
```

Here, a Favorite record is created where `favoriteable_type` is set to
"Uses", and then validations are run on that record. The presence
validation on `favoriteable_type` is run which tries to access a "Uses"
model. But that model doesn't exist, so the test raises an error.

Now `validates_uniqueness_of` will set the *_type attribute to a
meaningful value. It still does this by calling #next on the first
record's value, but then it makes a new model that is simply an alias
for the original model. Hence, in our example, Uses would become a model
that is aliased to User.

In order not to clutter the object space, these special models are
created under the Shoulda::Matchers::ActiveModel::Uniqueness::TestModels
namespace, and are automatically removed once `validate_uniqueness_of` is
finished.
Looks like this one isn't getting unset.