def execute(url, query)
      errors = execute_multiple url, [request_body(query)]
      raise errors.first if errors.first
    end
def execute(url, query)
      errors = execute_multiple url, [request_body(query)]
      raise errors.first if errors.first
    end
Sends a POST request to the specified Lightstreamer URL with the given query params. If an error occurs then a
 {LightstreamerError} subclass will be raised.

 @param [String] url The URL to send the POST request to.
 @param [Hash] query The POST request's query params.