Make providing bucket id optional if auth returns bucket id

[SayakMukhopadhyay]

B2 returns the bucket id on authorisation if the auth key is for a particular bucket. Thus, one can store the bucket id in the instance too and use it if bucket id is not passed in a function call.

I am open to implementing this myself. Looking for feedback initially.

[cbess]

Please provide the documentation for this behavior from the B2 API.

[SayakMukhopadhyay]

This is described in https://www.backblaze.com/b2/docs/b2_authorize_account.html under the responses section.

[odensc]

Good idea - a PR would be nice :)

[SayakMukhopadhyay]

All right. I will work on this and hopefully will be creating a PR sometime next week.