URL-encode URLs

[BlackbitDevs]

Currently there is only one Utils::encode() method which gets used to encode URLs as well as textual content.
For example in

PrestaSitemapBundle/src/Sitemap/Url/UrlConcrete.php

Line 202 in 62bf18a

$xml = '<url><loc>' . Utils::encode($this->getLoc()) . '</loc>';

the <loc> gets encoded with

PrestaSitemapBundle/src/Sitemap/Utils.php

Line 40 in 62bf18a

return htmlspecialchars($string, ENT_QUOTES | ENT_SUBSTITUTE, 'UTF-8');

With the following code

$url = new \Presta\SitemapBundle\Sitemap\Url\UrlConcrete('http://example.org/umlaut_url_ä');
var_dump($url->toXml());

the generated XML is:

<url>
  <loc>http://example.org/umlaut_url_ä</loc>
</url>

Actually this is not a valid URL according to RFC3986 (also see https://stackoverflow.com/questions/1856785/characters-allowed-in-a-url for summary). Although such URLs work when being called in a browser because the browser URL-encodes it, this library should follow the RFC imho.

With this PR above code would generate this XML:

<url>
  <loc>http://example.org/umlaut_url_%C3%A4</loc>
</url>

This is exactly the same as when you call http://example.org/umlaut_url_ä in your browser and then copy-paste the URL to a text editor.