docs: def get_headers(self, cmd, msg_length, extra_headers):
          """Returns the headers string based on command to execute"""
          cmd_header = "%s %s" % (cmd, PROTOCOL_VERSION)
          len_header = "Content-length: %s" % msg_length
          headers = [cmd_header, len_header]
          if self.user:
              user_header = "User: %s" % self.user
              headers.append(user_header)
          if self.gzip:
              headers.append("Compress: zlib")
          if extra_headers is not None:
              for key in extra_headers:
                  if key.lower() != 'content-length':
                      headers.append("%s: %s" % (key, extra_headers[key]))
          headers.append('')
          headers.append('')
          return '\r\n'.join(headers)