Add conditional request support

[Tjorriemorrie]

The github recommends conditional requests:

Most responses return an ETag header. Many responses also return a Last-Modified header. You can use the values of these headers to make subsequent requests to those resources using the If-None-Match and If-Modified-Since headers, respectively. If the resource has not changed, the server will return a 304 Not Modified. Also note: making a conditional request and receiving a 304 response does not count against your Rate Limit, so we encourage you to use it whenever possible.

Can we add this support to receive 304s?