Fix character set used for dynamically loaded scripts.

Fix character set used for dynamically loaded scripts.

Reviewed by Weinig, Dan and Alexey.

Fix for:
    <rdar://problem/5333163> Safari can not display the mouse over pop menu on ChinaTimes News site correctly.
    <rdar://problem/5530048> [Safari]? :Leopard9A576: The typed CH characters displays as garbage in Sina website after reloading the webpage.
    <rdar://problem/5416588> All menus for chinese IBM site have wrong encoding

Use the same logic to determine the charset for a script loaded dynamically
as we do for a statically loaded script.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@28612 268f45cc-cd09-0410-ab3c-d52691b4dbfc