Add toString to http.common.HTTPRequest / HTTPResponse

I find myself having to do that in most of my project. This is pretty handy for debugging:

````
requestHTTP("http://www.example.org/",
	(scope req) { req.method = HTTPMethod.GET; logInfo("=> %s", req); },
	(scope res) { logInfo("<= %s", req) });
````

The only change in behaviour would be that, when tracing, the status phrase will be output as well, not only the version and code.