Add new configuration to merge cache hits

- Specifically intended to get around analytics requests with
different URLs each time, sometimes even just slightly different
hostnames!
Since the use case isn't clear, I'd lean towards a separate config option to avoid any unexpected behavior.  Since we're talking hypotheticals, I'd say it's also likely someone would want merged cache hits /unless/ the body is different, which wouldn't work with this implementation.  How about adding something like `merge_cache_ignore_body` and have it false by default?