Update one_time_password.rb (#67)

When using auto_increment it wouldn't save the model so when I had a different instance of the user the counter would never match.

lib/active_model/one_time_password.rb

@@ -49,7 +49,7 @@ def authenticate_otp(code, options = {})
           result = hotp.verify(code, otp_counter)
           if result && options[:auto_increment]
             self.otp_counter += 1
-            save if respond_to?(:new_record?) && !new_record?
+            save if respond_to?(:changed?) && !new_record?
           end
           result
         else
@@ -66,7 +66,7 @@ def otp_code(options = {})
         if otp_counter_based
           if options[:auto_increment]
             self.otp_counter += 1
-            save if respond_to?(:new_record?) && !new_record?
+            save if respond_to?(:changed?) && !new_record?
           end
           ROTP::HOTP.new(otp_column, digits: otp_digits).at(self.otp_counter)
         else