Allow JWT generation when using the Container credential type

Author: mheap

src/Client.php

@@ -14,6 +14,7 @@
 use Nexmo\Client\Credentials\Keypair;
 use Nexmo\Client\Credentials\OAuth;
 use Nexmo\Client\Credentials\SignatureSecret;
+use Nexmo\Client\Exception\Exception;
 use Nexmo\Client\Factory\FactoryInterface;
 use Nexmo\Client\Factory\MapFactory;
 use Nexmo\Client\Response\Response;
@@ -220,8 +221,12 @@ public static function authRequest(RequestInterface $request, Basic $credentials
      * @param array $claims
      * @return \Lcobucci\JWT\Token
      */
-    public function generateJwt($claims = []) {
-        return $this->credentials->generateJwt($claims);
+    public function generateJwt($claims = [])
+    {
+        if (method_exists($this->credentials, "generateJwt")) {
+            return $this->credentials->generateJwt($claims);
+        }
+        throw new Exception(get_class($this->credentials).' does not support JWT generation');
     }
 
     /**

src/Client/Credentials/Container.php

@@ -62,4 +62,8 @@ public function has($type)
         return isset($this->credentials[$type]);
     }
 
+    public function generateJwt($claims) {
+        return $this->credentials[Keypair::class]->generateJwt($claims);
+    }
+
 }