That's odd, as far as I know /sockwrite -n has always worked this way: it will only append a CRLF if the line doesn't already end with one. At least that's what the 6.35 help file says.
You can also send the two CRLFs at the end like so:
sockwrite -n $sockname GET /get.html HTTP/1.1
sockwrite $sockname host: www.mirc.com $str($crlf,2)
This works as you would expect.