Fix error “`Missing field `language`”

The language field on MessageEntity may be absent even if the entity's type is pre, since this is not supported in clients AFAIK. So I wrapped the corresponding field in an Option to fix the error in #32 (closed).

This is a potential breaking change. So I did a poor man's crater (or is it more correct to call it tboter? 😀️) run on these bots I could find on GitHub (except for bots using v0.2) and they compile just fine with this patch applied:

• https://github.com/yejingchen/jieba-bot
• https://github.com/vyatu/markov_bot
• https://github.com/eskimor/accounting-bot
• https://github.com/iovxw/jandan_pic_bot

So I think that the chance of breaking something is low and we can release it as v0.5.1.

Fixes #32 (closed).