Fix $request_uri replacement

[abetomo]

$request_uri may be headed by two slashes.

$uri_rel = '/path/no.php';
$request_uri = '/path/no.php/page';
echo str_replace( $uri_rel, '/', $request_uri );

// -> "//page"

If the same string exists in the path, it will not be replaced as expected.

$uri_rel = '/path';
$request_uri = '/path/path1/page';
echo str_replace( $uri_rel, '/', $request_uri );
    
// -> "//1/page"

So, we'll replace only the first one.