Add option to systematically activate HTTP 1.1 requests

When trying to get documents from an Alfresco server, I got 505
errors. I investigated this only to discover that node only emit
HTTP 1.1 requests under some particular conditions.

This commit add support for a new option named useHTTP11.
When this option is found, the HTTP client will specify HTTP 1.1
protocol version. This has fixed my alfresco connectivity problem.

Example:

var opts = {
    useHTTP11: true
};

httpClient.get("http://server/url", opts);