Additional performance tweak for BZ 68558

https://bz.apache.org/bugzilla/show_bug.cgi?id=68558

Author: markt-asf

java/org/apache/tomcat/util/http/MimeHeaders.java

@@ -488,7 +488,7 @@ public boolean hasMoreElements() {
     public String nextElement() {
         MessageBytes current = next;
         findNext();
-        return current.toString();
+        return current.toStringType();
     }
 }