Differing requests can be returned the same deduplicated response

[markmccaigue]

It's possible that some differing requests receive the same response.

For example, making requests to the same endpoint, but with a different If-Modified-Since header, can result in the second request receiving the response from the first. These requests are perhaps being deduplicated when they should not be.

Our application makes use of the If-Modified-Since and If-None-Match headers. Looking at the UniqueKeyForRequest method, it appears that these headers are not taken into consideration. It also appears that non-idempotent requests such as POSTs may also be deduplicated, when a client may expect all of these requests to be send and responded to individually as state changes on the server.

Does this assessment seem on the right track, and would you accept a PR for these changes? Thanks!