fix for #1884 (dont cache POST)

What are the thoughts on this direction? 

There does not seem to be any tests for CacheDispatcher :-1:, any reason?

(refs #1884 not sure how to push direct to an issue)
@markstory would that be extendible? As it stands someone could make DELETE cacheable if they wanted (for example).

I am thinking you are talking about something like if (in_array($method, array(fixed list)) .. type check