Updates for Bot API 5.2, other small fixes

[Syfaro]

Changes:

• Changes for Bot API 5.2
• Add omitempty to some fields that should have it

Other notes:

• The provider_data field is documented as 'A JSON-serialized data about the invoice, which will be shared with the payment provider.' Is there some way to make this clearer with library types? It's currently a string field which seems odd (but that's what it says in the Bot API docs).
• The docs updated to say record_audio and upload_audio should have been record_voice and upload_voice, is there any better way to handle this than commenting that the previous values are deprecated?

[tjhorner]

LGTM!

[tjhorner]

• Is there some way to make this clearer with library types? It's currently a string field which seems odd (but that's what it says in the Bot API docs).

I think having a simple "See the payment API docs for more details" would suffice here. I assume if people need this field they know what they're doing anyway.

• The docs updated to say record_audio and upload_audio should have been record_voice and upload_voice, is there any better way to handle this than commenting that the previous values are deprecated?

I think this is the best way to go. I guess we could update the old constants to reflect the new values Telegram expects going forward, in case they ever remove support for the old values.