code: def merge_headers(event):
      
      headers = event.get('headers') or {}
      multi_headers = (event.get('multiValueHeaders') or {}).copy()
      for h in set(headers.keys()):
          if h not in multi_headers:
              multi_headers[h] = [headers[h]]
      for h in multi_headers.keys():
          multi_headers[h] = ', '.join(multi_headers[h])
      return multi_headers
docs: Merge the values of headers and multiValueHeaders into a single dict.
      Opens up support for multivalue headers via API Gateway and ALB.
      See: https://github.com/Miserlou/Zappa/pull/1756