Expires header is hardcoded for past date

[KrishnaPG]

drogon/lib/src/StaticFileRouter.cc

Lines 386 to 390 in 31d8519

	resp->addHeader("Last-Modified",
	fileStat.modifiedTimeStr_);
	resp->addHeader("Expires",
	"Thu, 01 Jan 1970 00:00:00 GMT");
	}

We want to mark our static files as cachable; Seems there is no way to achieve this correctly. The "static_files_cache_time" is not being respected, and the only thing that works is:

"app": {
	"mime": {
		"application/json": "json"
	},
	"static_file_headers": [
		{
			"name": "Cache-Control",
			"value": "public, max-age=86400, must-revalidate"
		}
	]

}

This sends valid cache-control headers in response, but this happens:

defeating the whole cache headers.

Also, the code seems to be supporting "if-modified-since"

drogon/lib/src/StaticFileRouter.cc

Lines 349 to 350 in 31d8519

	const std::string &modiStr = req->getHeaderBy("if-modified-since");
	if (enableLastModify_ && modiStr == fileStat.modifiedTimeStr_)

But it is not clear how to configure that "enableLastModify_" option from app() or in config;

[ericel]

#2515