I made the uri parameter optional when I introduced the origin
attribute, but on second thought it should always be given.
I'm making the origin parameter fixed and mandatory again, but this
time it comes next to set_cookie. This order should look more natural
because the one that comes first is to be parsed.
Since Mechanize::Cookie.parse required the uri parameter to be a URI
object, backward compatibility is still possible.
CookieJar#save is the new name for the now obsolete #save_as.
CookieJar#save and #load now accept IO-like instead of a filename.
Change the YAML file format, and make #load discard incompatible data.
