Ensure created_at isn't updated when unnecessary

- When we build the update whitelist (when the user doesn't provide one)
  we simply say that the whitelist is all columns except pkeys. The
  reason we do this is because sqlboiler doesn't support dirty tracking
  and if you want to update just a few fields, it's easier to just
  explicitly pass a whitelist when you update. However, in the case of
  created_at - it's a field that's usually managed by us and so we
  should not be updating it unless a user explicitly requests it via the
  whitelist. So if the whitelist is empty we prune that column now if it
  exists from the auto-created whitelist.
- The user also reported that upsert has the same fate, but the logic is
  somewhat different there. It expects that there's a value for
  created_at or it assumes that one needs to be created. There's no
  more magic we can do around this one unfortunately. Upsert is a
  mythical beast that doesn't play as nicely with our model, so users
  will just have to be more careful.
- Fix #106

templates/16_update.tpl

@@ -49,6 +49,11 @@ func (o *{{$tableNameSingular}}) Update(exec boil.Executor, whitelist ... string
 
 	if !cached {
 		wl := strmangle.UpdateColumnSet({{$varNameSingular}}Columns, {{$varNameSingular}}PrimaryKeyColumns, whitelist)
+		{{- if not .NoAutoTimestamps}}
+		if len(whitelist) == 0 {
+			wl = strmangle.SetComplement(wl, []string{"created_at"})
+		}
+		{{end -}}
 		if len(wl) == 0 {
 			return errors.New("{{.PkgName}}: unable to update {{.Table.Name}}, could not build whitelist")
 		}