Account for current age in cache calculations

Otherwise 304 responses get thrown off as they have the orig 200 date with the age set

src/index.js

@@ -45,6 +45,11 @@ class Onionoo {
     let maxAge = headers['cache-control'] && headers['cache-control'].match(/max-age=(\d+)/)
     maxAge = parseInt(maxAge ? maxAge[1] : 0)
 
+    // Take current age into account
+    if (headers.age) {
+      maxAge -= headers.age
+    }
+
     // Calculate expirey date
     return addSeconds(new Date(originDate), maxAge)
   }