Don't update the ratelimits if we got a response from a cache

The recommended cache, https://github.com/gregjones/httpcache, returns
an X-From-Cache: 1 header whenever it serves a response from the cache.

Cached responses might include (possibly stale) ratelimit headers, which
we would've already processed in the past.

github/github.go

@@ -653,9 +653,13 @@ func (c *Client) BareDo(ctx context.Context, req *http.Request) (*Response, erro
 
 	response := newResponse(resp)
 
-	c.rateMu.Lock()
-	c.rateLimits[rateLimitCategory] = response.Rate
-	c.rateMu.Unlock()
+	// Don't update the rate limits if this was a cached response.
+	// X-From-Cache is set by https://github.com/gregjones/httpcache
+	if response.Header.Get("X-From-Cache") == "" {
+		c.rateMu.Lock()
+		c.rateLimits[rateLimitCategory] = response.Rate
+		c.rateMu.Unlock()
+	}
 
 	err = CheckResponse(resp)
 	if err != nil {