fix: Fix function spec for Response.decode/2

The input type for `GoogleApi.Gax.Response.decode/2` was missing the error
case (which is explicitly handled in the first function clause).
The return type incorrectly specifies that the error type must be a
`Tesla.Env.t()` when in fact it may return any type (for example `:timeout`
is returned when the Mint HTTP client times out).

Author: bernardd

clients/gax/lib/google_api/gax/response.ex

@@ -33,9 +33,9 @@ defmodule GoogleApi.Gax.Response do
   ## Returns
 
   *   `{:ok, struct()}` on success
-  *   `{:error, Tesla.Env.t}` on failure
+  *   `{:error, any()}` on failure
   """
-  @spec decode({:ok, Tesla.Env.t()}, keyword()) :: {:ok, struct()} | {:error, Tesla.Env.t()}
+  @spec decode({:ok, Tesla.Env.t()} | {:error, any()}, keyword()) :: {:ok, struct()} | {:error, any()}
   def decode(env, opts \\ [])
 
   def decode({:error, reason}, _), do: {:error, reason}