Netty version bump breaks unit tests

[pietrygamat]

The recent bump of netty version (#172 ) breaks the unit tests in org.littleshoot.proxy.impl.ProxyUtilsTest#testCommaSeparatedHeaderValues .
It is not immediately clear to me if the tests are in fact obsolete and need updating, or is there some sort of bug in the updated library, so this issue should be examined closer.
In the meantime however I believe the revert of the offending library is in order: (#178)

[valfirst]

I've filed Netty issue: netty/netty#13005. Let's see how it goes.