Fix account.refresh to work even the second time

Previously, refreshing the token would not work the second time
because the @refreshed_authentications was caching the old token.

Gemfile.lock

@@ -1,7 +1,7 @@
 PATH
   remote: .
   specs:
-    yt (0.6.0)
+    yt (0.6.1)
       activesupport
 
 GEM

README.md

@@ -326,7 +326,7 @@ To install on your system, run
 
 To use inside a bundled Ruby project, add this line to the Gemfile:
 
-    gem 'yt', '~> 0.6.0'
+    gem 'yt', '~> 0.6.1'
 
 Since the gem follows [Semantic Versioning](http://semver.org),
 indicating the full version in your Gemfile (~> *major*.*minor*.*patch*)

lib/yt/associations/authentications.rb

@@ -25,9 +25,8 @@ def authentication_url
       # Returns true if the new access token is different from the previous one
       def refresh
         old_access_token = authentication.access_token
-        authentication.expire
-        new_access_token = authentication.access_token
-        old_access_token != new_access_token
+        @authentication = @access_token = @refreshed_authentications = nil
+        old_access_token != authentication.access_token
       end
 
     private

lib/yt/models/authentication.rb

@@ -13,10 +13,6 @@ def expired?
         @expires_at && @expires_at.past?
       end
 
-      def expire
-        @expires_at = 10.years.ago
-      end
-
     private
 
       def expiration_date(options = {})

lib/yt/version.rb

@@ -1,3 +1,3 @@
 module Yt
-  VERSION = '0.6.0'
+  VERSION = '0.6.1'
 end

spec/associations/device_auth/authentications_spec.rb

@@ -4,6 +4,19 @@
 describe Yt::Associations::Authentications, :device_app do
   subject(:account) { Yt::Account.new attrs }
 
+  describe '#refresh' do
+    context 'given a valid refresh token' do
+      let(:attrs) { {refresh_token: ENV['YT_TEST_DEVICE_REFRESH_TOKEN']} }
+
+      # NOTE: When the token is refreshed, YouTube *might* actually return
+      # the *same* access token if it is still valid. Typically, within the
+      # same second, refreshing the token returns the same token. Still,
+      # testing that *expires_at* changes is a guarantee that we attempted
+      # to get a new token, which is what refresh is meant to do.
+      it { expect{account.refresh}.to change{account.expires_at} }
+    end
+  end
+
   describe '#authentication' do
     context 'given a refresh token' do
       let(:attrs) { {refresh_token: refresh_token} }