Fix 'connection' header, copying of headers, and support Windows named pipes

This pull request is to replace #260 (https://github.com/nodejitsu/node-http-proxy/pull/260)

It contains the same changes + TESTS:
1. Set the connection header only if not already set by the target server.
2. Copy response headers case-insensitively

In addition it contains another fix:
support unix domain sockets and windows named pipes (socketPath) - This was transparent in node 0.6.x by using port for named pipe, but in node 0.8.x it should be passed explicitly.