If subsequent requests want json output (options.json = true), then we try
to JSON.parse() the cached content, which is already json, which throws.
This causes the cache lookup to fail, so we go get from source again.

If subsequent requests don't want json output, then we successfully serve
from the cache, but the cached content is json, rather than the expected
string content.

We JSON.parse and JSON.stringify the cached results as necessary.

Previously, if any request handler modified a json response, then that
modification would remain in the cache and be incorrectly returned in
subsequent requests.

With this change, we deep clone cached objects so each request handler gets
its own copy of the original response.

Cpacey/json fixes plus