MINOR: mux-h2: make h2c_send_rst_stream() use the dummy stream's error code

MINOR: mux-h2: make h2c_send_rst_stream() use the dummy stream's error code

We currently have 2 dummy streams allowing us to send an RST_STREAM
message with an error code matching this one. However h2c_send_rst_stream()
still enforces the STREAM_CLOSED error code for these dummy streams,
ignoring their respective errcode fields which however are properly
set.

Let's make the function always use the stream's error code. This will
allow to create other dummy streams for different codes.