net/http/client.go: fix cookie handling on (*Client) Do()

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

src/pkg/net/http/client_test.go

@@ -285,6 +285,10 @@ func TestClientSendsCookieFromJar(t *testing.T) {
 	req, _ := NewRequest("GET", us, nil)
 	client.Do(req) // Note: doesn't hit network
 	matchReturnedCookies(t, expectedCookies, tr.req.Cookies())
+
+	req, _ = NewRequest("POST", us, nil)
+	client.Do(req) // Note: doesn't hit network
+	matchReturnedCookies(t, expectedCookies, tr.req.Cookies())
 }
 
 // Just enough correctness for our redirect tests. Uses the URL.Host as the