Wrap discard methods in a transaction

This seeks to solve issue: jhawthorn#77

Given the situation where a `before/after` callback fails, particularly
with related records, we shouldn't still discard records.

Note: This is leveraging a private API that exists in
[ActiveRecord::Transactions](https://github.com/rails/rails/blob/cd2949d2d936daa89b7e23f816f1c004aee85461/activerecord/lib/active_record/transactions.rb#L345)

lib/discard/model.rb

@@ -117,9 +117,15 @@ def undiscarded?
     # @return [Boolean] true if successful, otherwise false
     def discard
       return false if discarded?
-      run_callbacks(:discard) do
-        update_attribute(self.class.discard_column, Time.current)
+
+      _in_discard_transaction do
+        run_callbacks(:discard) do
+          update_attribute(self.class.discard_column, Time.current)
+        end
       end
+
+    rescue Exception => e
+      handle_transaction_rollback(e)
     end
 
     # Discard the record in the database
@@ -131,16 +137,21 @@ def discard
     # @return [Boolean] true if successful
     # @raise {Discard::RecordNotDiscarded}
     def discard!
-      discard || _raise_record_not_discarded
+      with_transaction_returning_status do
+        discard || _raise_record_not_discarded
+      end
     end
 
     # Undiscard the record in the database
     #
     # @return [Boolean] true if successful, otherwise false
     def undiscard
       return unless discarded?
-      run_callbacks(:undiscard) do
-        update_attribute(self.class.discard_column, nil)
+
+      _in_discard_transaction do
+        run_callbacks(:undiscard) do
+          update_attribute(self.class.discard_column, nil)
+        end
       end
     end
 
@@ -158,6 +169,18 @@ def undiscard!
 
     private
 
+    def _in_discard_transaction
+      previous_discard_value = public_send(self.class.discard_column)
+
+      with_transaction_returning_status do
+        yield
+      end
+    rescue Exception => exception
+      assign_attributes(self.class.discard_column => previous_discard_value)
+
+      raise exception
+    end
+
     def _raise_record_not_discarded
       raise ::Discard::RecordNotDiscarded.new("Failed to discard the record", self)
     end