code: def test_user_agent_custom(self):
      geocoder = OpenCage(
          api_key="DUMMYKEY1234", user_agent="my_user_agent/1.0"
      )
      assert geocoder.headers["User-Agent"] == "my_user_agent/1.0"
  
file_path: geopy/test/geocoders/opencage.py
imports: import pytest
  from geopy.exc import GeocoderInsufficientPrivileges, GeocoderQuotaExceeded, GeocoderRateLimited
  from geopy.geocoders import OpenCage
  from test.geocoders.util import BaseTestGeocoder, env
label_desc: Use image node_id 1 for calling the TestUnitOpenCage obj's underlying member method code with example usage: obj.test_user_agent_custom() without return types