Fail when no scopes on uniq matcher but scopes on validation

Change behavior of `validate_uniqueness_of` when the matcher is not
qualified with any scopes, but your validation is. Previously the
following test would pass when it now fails:

    class Post < ActiveRecord::Base
      validate :slug, uniqueness: { scope: :user_id }
    end

    describe Post do
      it { should validate_uniqueness_of(:slug) }
    end

NEWS.md

@@ -29,6 +29,20 @@
   (which is to permit this) is misleading, as the test that you write by using
   `allow_value` is different from the test that actually ends up getting run.
 
+* Change behavior of `validate_uniqueness_of` when the matcher is not
+  qualified with any scopes, but your validation is. Previously the following
+  test would pass when it now fails:
+
+  ``` ruby
+  class Post < ActiveRecord::Base
+    validate :slug, uniqueness: { scope: :user_id }
+  end
+
+  describe Post do
+    it { should validate_uniqueness_of(:slug) }
+  end
+  ```
+
 ### Bug fixes
 
 * So far the tests for the gem have been running against only SQLite. Now they

lib/shoulda/matchers/active_record/validate_uniqueness_of_matcher.rb

@@ -259,7 +259,8 @@ def matches?(subject)
           @expected_message ||= :taken
           @all_records = @subject.class.all
 
-          set_scoped_attributes &&
+          scopes_match? &&
+            set_scoped_attributes &&
             validate_everything_except_duplicate_nils_or_blanks? &&
             validate_case_sensitivity? &&
             validate_after_scope_change? &&
@@ -271,6 +272,37 @@ def matches?(subject)
 
         private
 
+        def validation
+          @subject.class._validators[@attribute].detect do |validator|
+            validator.is_a?(::ActiveRecord::Validations::UniquenessValidator)
+          end
+        end
+
+        def scopes_match?
+          expected_scopes = Array.wrap(@options[:scopes])
+
+          if validation
+            actual_scopes = Array.wrap(validation.options[:scope])
+          else
+            actual_scopes = []
+          end
+
+          if expected_scopes == actual_scopes
+            true
+          else
+            @failure_message = "Expected validation to be scoped to " +
+              "#{expected_scopes}"
+
+            if actual_scopes.present?
+              @failure_message << ", but it was scoped to #{actual_scopes}."
+            else
+              @failure_message << ", but it was not scoped to anything."
+            end
+
+            false
+          end
+        end
+
         def allows_nil?
           if @options[:allow_nil]
             ensure_nil_record_in_database

spec/unit/shoulda/matchers/active_record/validate_uniqueness_of_matcher_spec.rb

@@ -113,11 +113,11 @@
         expect(record).not_to validate_uniqueness
       end
 
-      it 'accepts if the scope is unset beforehand' do
+      it 'rejects if the scope is unset beforehand' do
         record = build_record_validating_uniqueness(
           scopes: [ build_attribute(name: :scope, value: nil) ]
         )
-        expect(record).to validate_uniqueness
+        expect(record).not_to validate_uniqueness
       end
     end
 
@@ -130,8 +130,8 @@
       end
     end
 
-    define_method(:build_attribute) do |options|
-      options.merge(
+    define_method(:build_attribute) do |attribute_options|
+      attribute_options.merge(
         column_type: column_type,
         value_type: value_type,
         array: array