Fixes a bug that donwloading a file using wget or curl always fails even when it succeeds.

## Purpose
To resolve #443 .

## Changes
Use `Utils::system` method to download a file when wget or curl is used in `DownloadTask`.

## Effect
Users who don't enable curl extension can download files correctly.

## Tests
Add some test cases for `UrlDownloaderTest`.
`system('wget --no-check-certificate -c -O ' . $targetFilePath . ' ' . $url) !== false` must be inverted because `system` function returns `false` only when it fails.