Add support for encoding and language in header parameters

[thekid]

This RFC extend the web.Headers class to handles encoding in parameterized headers, implementing #132

Example

Reading:

use web\Headers;

$header= "attachment; filename=uber.txt; filename*=utf-8''%C3%BC.txt";

$disposition= Headers::parameterized()->parse($header);
$disposition->value();                // "attachment"
$disposition->param('filename');      // "über.txt"
$disposition->equivalent('filename'); // "uber.txt"

Writing:

use web\{Response, Parameterized};

// When both filename and filename* are present in a single header field 
// value, filename* is preferred over filename when both are understood. 
// It's recommended to include both for maximum compatibility
$response= new Response(/* ... */);
$response->header('Content-Disposition', new Parameterized('attachment')
  ->with('filename', 'über.txt', equivalent: 'uber.txt')
);

See also

• https://www.rfc-editor.org/info/rfc8187/
• https://developer.mozilla.org/en-US/docs/Web/HTTP/Reference/Headers/Content-Disposition