add PlainURL option for HttpRequest.

In case no need to encode the URL and it should be processed by developer manually.

It's very important for some APIs, i.e. 
There is a '/' in the path as a param, the server is expected: "http://www.xxx.com/api/%2f/user"
but it will not be encoded by java.net.URL if you just put "http://www.xxx.com/api///user",
and it will be double encoded as "http://www.xxx.com/api/%252F/user" if you put "http://www.xxx.com/api/%2F/user".