Don't cache requests without a SID header (#1056)

Prevents unauthenticated requests becoming cached for all users. The
Request-Token is not as random as we thought. It is calculated by

`(CurrentRequestId + 1) & 0xFFFFFF | (UnixTime(DateTime.UtcNow) << 24)`

which could mean that two clients making a series of unauthenticated
requests at the same time could receive the same token, i.e. it is not
actually pseudo- random as originally assumed.

DragaliaAPI/DragaliaAPI/Infrastructure/OutputCaching/RepeatedRequestPolicy.cs

@@ -76,6 +76,12 @@ private static bool GetShouldCache(OutputCacheContext context)
             return false;
         }
 
+        // We do not want to cache requests globally - make sure this request can be keyed to a particular user.
+        if (!request.Headers.ContainsKey(Headers.SessionId))
+        {
+            return false;
+        }
+
         return true;
     }
 }