+ Creates request on the underlying [HTTPClient]. If there is no configuration errors, it tries to connect using [method HTTPClient.connect_to_host] and passes parameters onto [method HTTPClient.request].
+ Returns [code]OK[/code] if request is successfully created. (Does not imply that the server has responded), [code]ERR_UNCONFIGURED[/code] if not in the tree, [code]ERR_BUSY[/code] if still processing previous request, [code]ERR_INVALID_PARAMETER[/code] if given string is not a valid URL format, or [code]ERR_CANT_CONNECT[/code] if not using thread and the [HTTPClient] cannot connect to host.