Avoid cached response from proxy

Trying to fetch the Yahoo Finance! homepage multiple times can result
in getting a cached response from a proxy. The cached response does
not have a "Set-cookie" header, so the curl handle will still not have
a cookie and the connection will be retried.

This StackOverflow Q/A has two approaches to avoid a cache hit:
https://stackoverflow.com/q/31653271/271616
1) Add a "Cache-control: no-cache" header
2) Add a random query to the URL.

The first approach didn't work because it appears the Yahoo proxies
simply ignore the request. The second approach did work though.

Fixes #166.

R/getSymbols.R

@@ -227,7 +227,10 @@ formals(loadSymbols) <- loadSymbols.formals
 
       for (i in 1:5) {
         h <- curl::new_handle()
-        curl::curl_download("https://finance.yahoo.com", tmp, handle = h)
+        # random query to avoid cache
+        ru <- paste(sample(c(letters, 0:9), 4), collapse = "")
+        cu <- paste0("https://finance.yahoo.com?", ru)
+        curl::curl_download(cu, tmp, handle = h)
         if (NROW(curl::handle_cookies(h)) > 0)
           break;
         Sys.sleep(0.1)