[Slack] Remove Slack's default scopes to avoid conflicts (#53)

* Remove Slack's default scopes to avoid conflicts

* Provide some default scopes for the Slack provider if the user didn't define some

Provider.php

@@ -17,18 +17,27 @@ class Provider extends AbstractProvider implements ProviderInterface
      */
     const IDENTIFIER = 'SLACK';
 
-    /**
-     * {@inheritdoc}
-     */
-    protected $scopes = ['identity.basic', 'identity.email', 'identity.team', 'identity.avatar'];
-
     /**
      * The separating character for the requested scopes.
      *
      * @var string
      */
     protected $scopeSeparator = ',';
 
+    /**
+     * {@inheritdoc}
+     */
+    public function getScopes()
+    {
+        if (count($this->scopes) > 0) {
+            return $this->scopes;
+        }
+
+        // Provide some default scopes if the user didn't define some.
+        // See: https://github.com/SocialiteProviders/Providers/pull/53
+        return ['identity.basic', 'identity.email', 'identity.team', 'identity.avatar'];
+    }
+
     /**
      * Middleware that throws exceptions for non successful slack api calls
      * "http_error" request option is set to true.