Add support for the new Tesla version

We updated the Tesla on our project to 1.6, and apparently this operation broke the ExForce dependency. We started to receive a decode error when executing the function ExForce.OAuth.get_token/2:

Tried with the version 1.5 as well and didn't worked. Tesla 1.4 worked as expected.
Apparently the problem is with some change made in Tesla after the version 1.5.0.