api: adds no-cache header for GET requests

Fixes #5356. Internet Explorer aggressively caches GET requests which
means that all API calls fetching data are cached. This fix adds a
Cache-Control header with the value no-cache to all GET requests to
the API.

pkg/api/http_server.go

@@ -171,6 +171,8 @@ func (hs *HttpServer) newMacaron() *macaron.Macaron {
 		m.Use(middleware.ValidateHostHeader(setting.Domain))
 	}
 
+	m.Use(middleware.AddDefaultResponseHeaders())
+
 	return m
 }