Use oauthAccessToken instead of IDToken when returnIDPCredential is yes

Another fix for #2522

Author: Yue-Wang-Google

Firebase/Auth/Source/FIRUser.m

@@ -1115,7 +1115,7 @@ - (void)linkAndRetrieveDataWithCredential:(FIRAuthCredential *)credential
           // Update the new token and refresh user info again.
           self->_tokenService = [[FIRSecureTokenService alloc]
               initWithRequestConfiguration:requestConfiguration
-                               accessToken:response.IDToken
+                               accessToken:response.oauthAccessToken
                  accessTokenExpirationDate:response.approximateExpirationDate
                               refreshToken:response.refreshToken];
           [self internalGetTokenWithCallback:^(NSString *_Nullable accessToken,