Instance url is no longer required for JWT auth

src/Omniphx/Forrest/Authentications/OAuthJWT.php

@@ -6,6 +6,7 @@
 use Firebase\JWT\JWT;
 use Omniphx\Forrest\Client as BaseAuthentication;
 use Omniphx\Forrest\Interfaces\AuthenticationInterface;
+use Omniphx\Forrest\Exceptions\MissingTokenException;
 
 class OAuthJWT extends BaseAuthentication implements AuthenticationInterface
 {
@@ -21,12 +22,21 @@ public static function getJWT($iss, $aud, $sub, $privateKey)
         return JWT::encode($payload, $privateKey, 'RS256');
     }
 
+    private function getDefaultInstanceURL()
+    {
+        if (isset($this->settings['instanceURL']) && !empty($this->settings['instanceURL'])) {
+            return $this->settings['instanceURL'];
+        } else {
+            return $this->credentials['loginURL'];
+        }
+    }
+
     public function authenticate($fullInstanceUrl = null)
     {
-        $loginUrl = $this->credentials['loginURL'];
-        $fullInstanceUrl = $fullInstanceUrl ?? $loginUrl . '/services/oauth2/token';
+        $fullInstanceUrl = $fullInstanceUrl ?? $this->getDefaultInstanceURL() . '/services/oauth2/token';
 
         $consumerKey = $this->credentials['consumerKey'];
+        $loginUrl = $this->credentials['loginURL'];
         $username = $this->credentials['username'];
         $privateKey = $this->credentials['privateKey'];