Initial json requests cause subsequent requests to behave unexpectedly

[cpacey]

These only happen if the initially-cached request has options.json = true.

1. 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.
2. If subsequent requests don't want json output (options.json = false), then we successfully serve from the cache, but the cached content is json, rather than the expected string content.

I've written some tests demonstrating these behaviours.