Omit the unnecessary fields in toggle block

[yoyomolinas]

The ToggleBlock specifies RichText and Text fields which are not documented in the Notion API. I suppose these were left from earlier versions.

Practically, these fields should not be causing any errors as they're serialised only when they're present (i.e. omitempty). However, the RichText field misses the omitempty, causing error responses form Notion.

Would be great if the unnecessary fields can be cleaned up - if that's too much work just adding an omitempty tag to ToggleBlock.RichText would do the job too.

Ref. https://github.com/jomei/notionapi/blob/main/block.go#L352.

PS, not sure how you manage contributions here, but given the right permissions I can open up a PR as well.

Many thanks.

[jomei]

Fixed in #102