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!
The cukes that I'm currently using this for don't currently POST to analytics. Although when I thought it through, I wanted to handle this case for 2 reasons:
1. It's not inconceivable that an analytics library could construct a POST request. Potentially, in their favour, POST requests aren't cached, so they could guarantee hits every time by doing that.
2. Even if no analytics library ever does, there may be other use cases for the "merge_cached_responses_whitelist" feature that don't involve analytics (quite likely in my opinion). Without this check, it will break for POSTs.