extend request headers if the corresponding options object is set 

Sets the header options of the request. e.g.:

    var connect = require('connect');
    var url = require('url');
    var proxy = require('proxy-middleware');
    var app = connect();
    var options = url.parse('http://sub.example.com/api');
    options.headers = {host: 'my.example.com'};
    app.use('/api', proxy(options));
    app.use('/', connect['static']('www'));
    app.listen(8000);

In this case, assuming you are running the server @ `localhost:8000`,
the request to `http://sub.example.com/api` will have the `host` header
set to `my.example.com` instead of `localhost`.