Disallow changing the origin of a cookie once it is set.

lib/http/cookie.rb

@@ -220,6 +220,8 @@ class HTTP::Cookie
   end
 
   def origin=(origin)
+    @origin.nil? or
+      raise ArgumentError, "origin cannot be changed once it is set"
     origin = URI(origin)
     acceptable_from_uri?(origin) or
       raise ArgumentError, "unacceptable cookie sent from URI #{origin}"

test/test_http_cookie.rb

@@ -505,6 +505,9 @@ class TestHTTPCookie < Test::Unit::TestCase
     assert_equal '/', cookie.path
     assert_equal 'example.com', cookie.domain
     assert_equal true, cookie.for_domain
+    assert_raises(ArgumentError) {
+      cookie.origin = URI.parse('http://www.example.com/')
+    }
 
     cookie_str = 'a=b; domain=example.com'
     cookie = HTTP::Cookie.parse(cookie_str).first