libwebsockets

fix issuing null extension header if none supported

Josh realized that with new Chrome, because we don't support the type of
compression extension yet we returned a null extension header.

This patch fixes that by deferring issuing the extension header until we
find we have something to say.

tested OK on google-chrome 19.0.1081.2-129295

Reported-by: Josh Roberson <josh@asteriasgi.com>
Signed-off-by: Andy Green <andy.green@linaro.org>