Make OctoPrint.upload's url behave like others

So far only was interpreted relatively to the defined base url.
We want this to behave like other requests from the client lib
though, so now it does.

src/octoprint/static/js/app/client/base.js

@@ -222,7 +222,12 @@
 
         var headers = OctoPrint.getRequestHeaders();
 
-        request.open("POST", OctoPrint.getBaseUrl() + url);
+        var urlToCall = url;
+        if (!_.startsWith(url, "http://") && !_.startsWith(url, "https://")) {
+            urlToCall = OctoPrint.getBaseUrl() + url;
+        }
+
+        request.open("POST", urlToCall);
         _.each(headers, function(value, key) {
             request.setRequestHeader(key, value);
         });