Fix `rejectUnknownFields` to respect `fieldLabelModifier`
* Apply `fieldLabelModifier` to known fields reflected from the `FieldName` class. While NOT applying the `fieldLabelModofier` to the encoding tags. * Change the intermediary type returned by the `FieldName` class from `Text` to `String` to reduce `{un,}pack` calls to a minimum. * Update tests which specified the problem before to assert the fixed semantics.
