Make Charset actually return/take string, not CSSString as documented

Author: sabberworm

src/CSSList/CSSList.php

@@ -128,7 +128,7 @@ private static function parseListItem(ParserState $oParserState, CSSList $oList)
                         $oParserState->currentLine()
                     );
                 }
-                $oParserState->setCharset($oAtRule->getCharset()->getString());
+                $oParserState->setCharset($oAtRule->getCharset());
             }
             return $oAtRule;
         } elseif ($oParserState->comes('}')) {

src/Property/Charset.php

@@ -4,6 +4,7 @@
 
 use Sabberworm\CSS\Comment\Comment;
 use Sabberworm\CSS\OutputFormat;
+use Sabberworm\CSS\Value\CSSString;
 
 /**
  * Class representing an `@charset` rule.
@@ -16,9 +17,9 @@
 class Charset implements AtRule
 {
     /**
-     * @var string
+     * @var CSSString
      */
-    private $sCharset;
+    private $oCharset;
 
     /**
      * @var int
@@ -31,12 +32,12 @@ class Charset implements AtRule
     protected $aComments;
 
     /**
-     * @param string $sCharset
+     * @param CSSString $oCharset
      * @param int $iLineNo
      */
-    public function __construct($sCharset, $iLineNo = 0)
+    public function __construct(CSSString $oCharset, $iLineNo = 0)
     {
-        $this->sCharset = $sCharset;
+        $this->oCharset = $oCharset;
         $this->iLineNo = $iLineNo;
         $this->aComments = [];
     }
@@ -50,21 +51,22 @@ public function getLineNo()
     }
 
     /**
-     * @param string $sCharset
+     * @param string|CSSString $oCharset
      *
      * @return void
      */
     public function setCharset($sCharset)
     {
-        $this->sCharset = $sCharset;
+        $sCharset = $sCharset instanceof CSSString ? $sCharset : new CSSString($sCharset);
+        $this->oCharset = $sCharset;
     }
 
     /**
      * @return string
      */
     public function getCharset()
     {
-        return $this->sCharset;
+        return $this->oCharset->getString();
     }
 
     /**
@@ -96,7 +98,7 @@ public function atRuleName()
      */
     public function atRuleArgs()
     {
-        return $this->sCharset;
+        return $this->oCharset;
     }
 
     /**