Add headers from proxy config to WebSocket requests

[Aletheios]

For my project I have to add an additional header (authorization) to proxied WebSocket requests and noticed that it didn't seem to be supported yet. Usage:

{
    context: '/foobar',
    host: 'host',
    port: 1234,
    headers: {
        Authorization: 'foobar'
    },
    ws: true
}

[sebastienroul]

@Aletheios , I sent pull request #149 which fix the problem, like it :)