[v2.05] adds observers for cached and partially-cached responses

https://developer.mozilla.org/en-US/docs/Observer_Notifications#HTTP_requests
* 'http-on-examine-cached-response'
* 'http-on-examine-merged-response'

I was running a test using a response rule that normalizes 'content-type'
values that end with a "+json" suffix,
changing the value to: 'application/json'

for example:
============
* url            = http://json-ld.org/contexts/person.jsonld
* 'content-type' = 'application/ld+json'

..and I noticed some odd behavior:
* every other page load would work, and
* every other page load would fail

figured it must be a caching issue, and sure-enough..
this commit fixes the problem.