Ensured Netty provider uses CONNECT when proxying ws

The WS standard requires that clients use the CONNECT method when
proxying WebSockets, even if they aren't using SSL.  The Grizzly
provider already supported this, but the Netty didn't.

I also implemented a test that tested this case.
Ensured relative URIs are used to connect to proxied WebSockets

This needed to be done both for the Grizzly and the Netty provider.

Since this is essentially the same configuration parameter as the
useRelativeURIsWithSSLProxies, I reused that parameter, but I renamed it
to useRelativeURIsWithConnectProxies to reflect its true meaning since
it's no longer used just for SSL, and I deprecated the old parameter.
Merge pull request #657 from jroper/websocket-proxy-connect

Ensured Netty provider uses CONNECT when proxying ws