fix: ensure adapter is set

[yordis]

Context

This ensures that the returned Tesla.Env includes the adapter used to make the call. This will help debug which adapter was used and also reflect what truly happened at runtime.

Extra

Before

iex(1)> Tesla.get("https://httpbin.org/get")
{:ok,
 %Tesla.Env{
   __client__: %Tesla.Client{
     adapter: nil
   }
 }}

After

iex(1)> Tesla.get("https://httpbin.org/get")
{:ok,
 %Tesla.Env{
   __client__: %Tesla.Client{
     adapter: {Tesla.Adapter.Httpc, :call, [[]]}
   }
 }}