Set user agent in getting HTTP status code

Close #8

src/configloader/ValidateInstanceLink.go

@@ -3,7 +3,6 @@ package configloader
 import (
     "errors"
     "net/url"
-    "net/http"
     "strings"
 )
 
@@ -29,11 +28,11 @@ func ValidateInstanceLink(instanceUrl string) (string, error) {
         Host: parsed.Host,
         Path: "/musdb",
     }
-    resp, err := http.Get(musdbUrl.String())
+    statusCode, err := HttpGetStatusCode(musdbUrl.String())
 	if err != nil {
 		return "", err
 	}
-	if resp.StatusCode > 399 {
+	if statusCode > 399 {
         return "", errors.New("The museum-digital subdomain does not contain a musdb path")
 	}