Default user agent of axios

[untitaker]

Any usage of new Mastodon(..) appears to use the axios user-agent instead of the megalodon user agent.

I believe that is because userAgent in this file is not actually ever passed to the axios API: https://github.com/h3poteto/megalodon/blob/master/megalodon/src/mastodon/api_client.ts

Disclaimer: I'm not a direct user of megalodon -- I'm trying to debug an application that uses it to see why it is getting blocked by my Mastodon instance, and when browsing through its and megalodon's codebase found this issue. I hope I understood the code right.