Expires Header Format added

Like your library.  Just wanted to contribute a convenience format to keep me from having to keep looking it up.  

I'm not sure if the expires header requires you submit the timezone as GMT (which is equivalent of course to UTC)  but I'm not sure if all browsers it requires GMT In the string in order to behave correctly.

Right now "expiresHeaderFormat" would produce a string ending in UTC but otherwise in the correct format assuming you executed it like.

dateFormat(now, "expiresHeader", true)