Fix the problem with no cookie handling when sending
other than GET or HEAD request through
(*Client) Do(*Request) (*Resposne, error).
https://code.google.com/p/go/issues/detail?id=3985

Adds a function (*Client) send(*Request) (*Reponse, error):
- sets cookies from CookieJar to request,
- sends request
- parses a reply cookies and updates CookieJar

Fixes #3985

R=bradfitz
CC=gobot, golang-dev
https://golang.org/cl/6653049