Experimental TSHttpTxnCacheLookupStatusSet(HIT_STALE) calls cause a crash

If you use the API TSHttpTxnCacheLookupStatusSet() which is in experimental.h but is potentially very handy, and you set the status to HIT_STALE TS asserts. The code where it does that seems to be assuming that it is talking to a parent proxy, but in our setup was talking to the origin server. The fix seems pretty simple to make it handle talking to a origin server.