Do not use method as argument for pass by reference on Token (microsoft#170)

Author: dantleech

src/Node/StringLiteral.php

@@ -30,7 +30,8 @@ public function getStringContentsText() {
         $stringContents = "";
         if (isset($this->startQuote)) {
             foreach ($this->children as $child) {
-                $stringContents .= $child->getFullText($this->getFileContents());
+                $contents = $this->getFileContents();
+                $stringContents .= $child->getFullText($contents);
             }
         } else {
             // TODO ensure string consistency (all strings should have start / end quote)