uniqueness: Extract code to gen dummy values

We need to use this in another part of the codebase: later, when we add
ignoring_interference_by_writer to all matchers, we will need a way of
overriding an attribute such that it produces another value that
invalidates the test. Some tests require us to generate dummy values.

lib/shoulda/matchers/active_record/validate_uniqueness_of_matcher.rb

@@ -591,20 +591,7 @@ def dummy_value_for(scope)
         end
 
         def dummy_scalar_value_for(column)
-          case column.type
-          when :integer
-            0
-          when :date
-            Date.today
-          when :datetime
-            DateTime.now
-          when :uuid
-            SecureRandom.uuid
-          when :boolean
-            true
-          else
-            'dummy value'
-          end
+          Shoulda::Matchers::Util.dummy_value_for(column.type)
         end
 
         def next_value_for(scope, previous_value)

lib/shoulda/matchers/util.rb

@@ -50,6 +50,29 @@ def self.inspect_values(values)
       def self.inspect_range(range)
         "#{inspect_value(range.first)} to #{inspect_value(range.last)}"
       end
+
+      def self.dummy_value_for(column_type, array: false)
+        if array
+          [dummy_value_for(column_type, array: false)]
+        else
+          case column_type
+          when :integer
+            0
+          when :date
+            Date.new(2100, 1, 1)
+          when :datetime, :timestamp
+            DateTime.new(2100, 1, 1)
+          when :time
+            Time.new(2100, 1, 1)
+          when :uuid
+            SecureRandom.uuid
+          when :boolean
+            true
+          else
+            'dummy value'
+          end
+        end
+      end
     end
   end
 end