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/middleware/middleware.go

@@ -245,3 +245,11 @@ func (ctx *Context) HasHelpFlag(flag m.HelpFlags1) bool {
 func (ctx *Context) TimeRequest(timer metrics.Timer) {
 	ctx.Data["perfmon.timer"] = timer
 }
+
+func AddDefaultResponseHeaders() macaron.Handler {
+	return func(ctx *Context) {
+		if ctx.IsApiRequest() && ctx.Req.Method == "GET" {
+			ctx.Resp.Header().Add("Cache-Control", "no-cache")
+		}
+	}
+}