add gz as application/octet-stream

Karma-runner uses text/plain when serving files with an unrecognized mime type according to node-mim. Unfortunately this causes weird problems with java applets (such as throwing a ClassNotFoundException after loading a jar.pack.gz file).

So below is a patch that adds gz to the octet stream list.

https://github.com/karma-runner/karma/blob/master/lib/middleware/common.js#L54