Hackney adapter is incompatible with with_body option

[christopheradams]

The Hackney adapter allows options to be passed directly to the underlying request function. However, one option that will not work is with_body: true, which returns the body directly, and is needed for the max_body option.

The reason it won't work is that Tesla always tries to read the body from a reference:

https://github.com/teamon/tesla/blob/33417c02d082801013eb4e8a4b3cd3fa78db0d91/lib/tesla/adapter/hackney.ex#L38-L39

Passing with_body: true as an option to Tesla using Hackney will crash with a :req_not_found adapter error.

I believe it might be sufficient to use is_reference/1 as a guard to know whether the body needs to be read or not.

[teamon]

Closing since #103 is now merged.
@christopheradams feel free to reopen if that does not solve this issue.