Add failing test for 503 Service Unavailable

spec/fixtures/slack/web/503_error.yml

@@ -0,0 +1,14 @@
+---
+http_interactions:
+- request:
+    method: post
+    uri: https://slack.com/api/auth.test
+  response:
+    status:
+      code: 503
+      message: Service Unavailable
+    body:
+      encoding: US-ASCII
+      string: ''
+    http_version:
+  recorded_at: Thu, 30 Nov 2017 14:36:26 GMT

spec/slack/web/api/errors/service_unavailable_spec.rb

@@ -0,0 +1,14 @@
+require 'spec_helper'
+
+RSpec.describe Slack::Web::Client do
+  let(:client) { Slack::Web::Client.new }
+  it 'raises a Faraday::ClientError when Slack is unavailable', vcr: { cassette_name: 'web/503_error' } do
+    begin
+      client.auth_test
+      raise 'Expected to receive Faraday::ClientError.'
+    rescue Faraday::ClientError => e
+      expect(e.response).to_not be_nil
+      expect(e.response.status).to eq 503
+    end
+  end
+end