[Bug] merge is not called on delete if defaultblock does not match the tool type

Today I added subtitle support,
this can simply be done extending paragraph tool.
I forked the tool and changed the name to 'Subtitle'
I added support of timeline in addition to text like this

```js
/**
* Get current Tools`s data
* @returns {SubtitleData} Current data
* @private
*/
get data() {
let text = this._element.innerHTML;

this._data.text = text;
this._data.sstart = this._element.dataset.start ? this._element.dataset.start : 0;
this._data.send = this._element.dataset.send ? this._element.dataset.end : 0;

return this._data;
}

/**
* Store data in plugin:
* - at the this._data property
* - at the HTML
*
* @param {SubtitleData} data — data to set
* @private
*/
set data(data) {
this._data = data || {};

this._element.innerHTML = this._data.text || '';
this._element.dataset.start=this._data.sstart;
this._element.dataset.end=this._data.send;
}
```

The problem I encounter is that delete that call merge did not work and does nothing, just going up
until the tool matches defaultBlock subtitle.

I think it is a bug because it should look at type of previous block type