FIX GITHUB OAUTH ISSUE

Author: Thomasr

server/api-service/lowcoder-server/src/main/java/org/lowcoder/api/authentication/request/oauth2/request/GenericAuthRequest.java

@@ -42,6 +42,7 @@ protected Mono<AuthToken> getAuthToken(OAuth2RequestContext context) {
                         .with("client_secret", config.getClientSecret())
                         .with("grant_type", "authorization_code")
                         .with("redirect_uri", context.getRedirectUrl()))
+                .accept(MediaType.APPLICATION_JSON)
                 .retrieve()
                 .bodyToMono(Map.class)
                 .flatMap(map -> {

server/api-service/lowcoder-server/src/main/java/org/lowcoder/api/authentication/util/AdvancedMapUtils.java

@@ -50,6 +50,6 @@ public static String getString(Map<String, Object> map, String key) {
             }
         }
 
-        return current instanceof String ? (String) current : null;
+        return current!=null?current.toString():null;
     }
 }