net/http: the Set-Cookie Expiration time zone should be GMT, not UTC

Per RFC 6265.

Change-Id: I2b6b145f5d057f96509332509d722602ed9e2bbd
Reviewed-on: https://go-review.googlesource.com/10711Reviewed-by: Brett Slatkin <bslatkin@google.com>
Reviewed-by: Andrew Gerrand <adg@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>

src/net/http/cookie.go

@@ -156,7 +156,7 @@ func (c *Cookie) String() string {
 		}
 	}
 	if c.Expires.Unix() > 0 {
-		fmt.Fprintf(&b, "; Expires=%s", c.Expires.UTC().Format(time.RFC1123))
+		fmt.Fprintf(&b, "; Expires=%s", c.Expires.UTC().Format(TimeFormat))
 	}
 	if c.MaxAge > 0 {
 		fmt.Fprintf(&b, "; Max-Age=%d", c.MaxAge)

src/net/http/cookie_test.go

@@ -52,6 +52,10 @@ var writeSetCookiesTests = []struct {
 		&Cookie{Name: "cookie-8", Value: "eight", Domain: "::1"},
 		"cookie-8=eight",
 	},
+	{
+		&Cookie{Name: "cookie-9", Value: "expiring", Expires: time.Unix(1257894000, 0)},
+		"cookie-9=expiring; Expires=Tue, 10 Nov 2009 23:00:00 GMT",
+	},
 	// The "special" cookies have values containing commas or spaces which
 	// are disallowed by RFC 6265 but are common in the wild.
 	{