correct type specs and add dialyzer configuration

- Update @specs in APIClient and Connection to match actual returns
- Add dialyzer config to handle generated code warnings

.dialyzer_ignore.exs

@@ -0,0 +1,3 @@
+[
+  ~r/lib\/docusign\/api\/.*/
+]

lib/docusign/api_client.ex

@@ -3,13 +3,14 @@ defmodule DocuSign.APIClient do
   Deprecated interface to manage OAuth2 clients. `DocuSign.ClientRegistry` should be used
   instead.
   """
+
   alias DocuSign.ClientRegistry
 
   @doc """
   Get Api Client
   """
   @deprecated "Use DocuSign.ClientRegistry.client/1 instead."
-  @spec client() :: OAuth2.Client.t()
+  @spec client() :: {:ok, OAuth2.Client.t()} | {:error, any()}
   def client do
     ClientRegistry.client(default_user_id())
   end
@@ -18,7 +19,7 @@ defmodule DocuSign.APIClient do
   Forces an access token refresh.
   """
   @deprecated "Use DocuSign.ClientRegistry.refresh_token/1 instead."
-  @spec refresh_token() :: OAuth2.Client.t()
+  @spec refresh_token() :: {:ok, OAuth2.Client.t()} | {:error, any()}
   def refresh_token do
     # TO BE IMPLEMENTED
     # ClientRegistry.refresh_token(default_user_id())

lib/docusign/connection.ex

@@ -130,7 +130,7 @@ defmodule DocuSign.Connection do
   @doc """
   Makes a request.
   """
-  @spec request(t(), Keyword.t()) :: OAuth2.Response.t()
+  @spec request(t(), Keyword.t()) :: {:ok, OAuth2.Response.t()} | {:error, Tesla.Env.t()}
   def request(conn, opts \\ []) do
     timeout = Application.get_env(:docusign, :timeout, @timeout)
     opts = opts |> Keyword.merge(opts: [adapter: [timeout: timeout]])

mix.exs

@@ -21,7 +21,11 @@ defmodule DocuSign.MixProject do
       homepage_url: @url,
       deps: deps(),
       elixirc_paths: elixirc_paths(Mix.env()),
-      docs: docs()
+      docs: docs(),
+      dialyzer: [
+        ignore_warnings: ".dialyzer_ignore.exs",
+        plt_add_apps: [:mix]
+      ]
     ]
   end