Merge pull request #1 from tatsumi33/fix-httpresponse-errorcheck

Add error check for HTTP response
diff --git a/digest_auth_client.go b/digest_auth_client.go
index b608ce0..a67a6b2 100644
--- a/digest_auth_client.go
+++ b/digest_auth_client.go
@@ -48,6 +48,10 @@
 		}
 		resp, err = client.Do(req)
 
+		if err != nil {
+			return nil, err
+		}
+
 		if resp.StatusCode == 401 {
 			return dr.executeNewDigest(resp)
 		}