Uniqueness: Fix default behavior for case-insensitive models

If you have a model that has a uniqueness validation using
`case_sensitive: false`, and you are testing against but do not qualify
the matcher with `case_insensitive`, then the matcher will pass
unexpectedly. Take this model for instance:

    class Product < ActiveRecord::Base
      validates_uniqueness_of :name, case_sensitive: false
    end

Currently, the following test passes. With this commit, it will now
(correctly) fail:

    describe Product do
      it { is_expected.to validate_uniqueness_of(:name) }
    end

lib/shoulda/matchers/active_record/validate_uniqueness_of_matcher.rb

@@ -244,6 +244,7 @@ def matches?(subject)
 
           set_scoped_attributes &&
             validate_everything_except_duplicate_nils_or_blanks? &&
+            validate_case_sensitivity? &&
             validate_after_scope_change? &&
             allows_nil? &&
             allows_blank?
@@ -353,6 +354,22 @@ def validate_everything_except_duplicate_nils_or_blanks?
           disallows_value_of(existing_value, @expected_message)
         end
 
+        def validate_case_sensitivity?
+          value = existing_value
+
+          if value.respond_to?(:swapcase)
+            swapcased_value = value.swapcase
+
+            if @options[:case_insensitive]
+              disallows_value_of(swapcased_value, @expected_message)
+            else
+              allows_value_of(swapcased_value, @expected_message)
+            end
+          else
+            true
+          end
+        end
+
         def create_record_with_value
           @existing_record = create_record_in_database
         end
@@ -462,11 +479,7 @@ def available_enum_values_for(scope, previous_value)
         end
 
         def existing_value
-          value = existing_record.__send__(@attribute)
-          if @options[:case_insensitive] && value.respond_to?(:swapcase!)
-            value.swapcase!
-          end
-          value
+          existing_record.__send__(@attribute)
         end
 
         def class_name

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

@@ -392,7 +392,7 @@
     end
   end
 
-  context 'when the model has a case-sensitive validation on a string attribute' do
+  context 'when the model has a case-sensitive validation' do
     context 'when case_insensitive is not specified' do
       it 'accepts' do
         record = build_record_validating_uniqueness(
@@ -416,6 +416,30 @@
     end
   end
 
+  context 'when the model has a case-insensitive validation' do
+    context 'when case_insensitive is not specified' do
+      it 'rejects' do
+        record = build_record_validating_uniqueness(
+          attribute_type: :string,
+          validation_options: { case_sensitive: false }
+        )
+
+        expect(record).not_to validate_uniqueness
+      end
+    end
+
+    context 'when case_insensitive is specified' do
+      it 'accepts' do
+        record = build_record_validating_uniqueness(
+          attribute_type: :string,
+          validation_options: { case_sensitive: false }
+        )
+
+        expect(record).to validate_uniqueness.case_insensitive
+      end
+    end
+  end
+
   context 'when the validation is declared with allow_nil' do
     context 'given a new record whose attribute is nil' do
       it 'accepts' do