transmit_error_response(): allow to transmit error even if response was set.
transmit_error_response(): allow to transmit error even if response was set.

ghaderer/libmicrohttpd,ghaderer/libmicrohttpd,maru/libmicrohttpd-http2,maru/libmicrohttpd-http2,maru/libmicrohttpd-http2,ghaderer/libmicrohttpd