Append writable bytes to the readable bytes.
void commit( std::size_t n);
Appends n bytes from the start of the writable bytes to the end of the
readable bytes. The remainder of the writable bytes are discarded. If n
is greater than the number of writable bytes, all writable bytes are appended
to the readable bytes. All buffer sequences previously obtained using
prepare are invalidated. Buffer
sequences previously obtained using
data remain valid.
The number of bytes to append. If this number is greater than the number of writable bytes, all writable bytes are appended.