HTTP version selection #987

class HttpClient
{
    //Version DefaultVersion { get; set; } // Exists today - will be interpreted as Min or Max based on new API below.
    public HttpVersionPolicy DefaultVersionPolicy { get; set; } = HttpVersionPolicy.RequestVersionOrLower;
}

class HttpRequestMessage
{
    //Version Version { get; set; } // Exists today - will be interpreted as Min or Max based on new API below.
    public HttpVersionPolicy VersionPolicy { get; set; } = HttpVersionPolicy.RequestVersionOrLower;
}

public enum HttpVersionPolicy
{
    RequestVersionOrLower,
    RequestVersionOrHigher,
    RequestVersionExact
}

var msg = new HttpRequestMessage { VersionPolicy = HttpVersionPolicy.RequestVersionOrHigher };
// ...

var msg = new HttpRequestMessage { Version = HttpVersion.Version30, VersionPolicy = VersionPolicy = HttpVersionPolicy.RequestVersionOrHigher };
// ...

var msg = new HttpRequestMessage { VersionPolicy = HttpVersionPolicy.RequestVersionOrHigher };

var msg = new HttpRequestMessage
{
    Version = HttpVersion.Version20,
    VersionPolicy = HttpVersionPolicy.RequestVersionOrHigher
    RequestUri = new Uri("http://unsecure-url.com")
};

var msg = new HttpRequestMessage { }; // default policy just fine.
msg.Headers.ConnectionClose = true;