Sanitizing the field key may cause unwanted behavior

The `inner_set_key()` method is used for all keys set through the `Field` constructor. It sanitizes keys with the WP `sanitize_title()` function. This is necessary when the key is created out of a label text, but it might not be wanted if the user sets the key explicitly.

Could the sanitizing be done only if the key is created out of a label text?