added request to obtain a new access token with refresh token.

src/OAuth2/AbstractProvider.php

@@ -116,6 +116,25 @@ protected function makeAccessTokenRequest(string $code): RequestInterface
         ;
     }
 
+    /**
+     * @param string $refreshToken
+     * @return RequestInterface
+     */
+    protected function makeRefreshAccessTokenRequest(string $refreshToken): RequestInterface
+    {
+        $parameters = [
+            'refresh_token' => $refreshToken,
+            'client_id' => $this->consumer->getKey(),
+            'client_secret' => $this->consumer->getSecret(),
+            'grant_type' => 'refresh_token',
+        ];
+
+        return $this->httpStack->createRequest($this->requestHttpMethod, $this->getRequestTokenUri())
+            ->withHeader('Content-Type', 'application/x-www-form-urlencoded')
+            ->withBody($this->httpStack->createStream(http_build_query($parameters, '', '&')))
+        ;
+    }
+
     /**
      * @param string $code
      * @return AccessToken
@@ -132,6 +151,24 @@ public function getAccessToken(string $code): AccessToken
         return $this->parseToken($response->getBody()->getContents());
     }
 
+
+    /**
+     * @param string $refreshToken
+     *
+     * @return AccessToken
+     * @throws InvalidAccessToken
+     * @throws InvalidResponse
+     * @throws \Psr\Http\Client\ClientExceptionInterface
+     */
+    public function refreshAccessToken(string $refreshToken): AccessToken
+    {
+        $response = $this->executeRequest(
+            $this->makeRefreshAccessTokenRequest($refreshToken)
+        );
+
+        return $this->parseToken($response->getBody()->getContents());
+    }
+
     /**
      * @param array $parameters
      * @return AccessToken