add a test case to verify cache breaks with etags

When a client sends requests to express with the correct cache headers it expects to get back a 304
response when the data hasn't changed.  Currently if the entry is in the
cache the client will recieve a 200 response with the data again and if
the cache has passed its ttl the server will respond with a 500 as it
fails to deal with the empty data object returned by express in res.end.

Express handles these headers correctly with out cacher.

Adding 2 test cases to verify this behaviour