This week's book giveaway is in the OO, Patterns, UML and Refactoring forum. We're giving away four copies of Refactoring for Software Design Smells: Managing Technical Debt and have Girish Suryanarayana, Ganesh Samarthyam & Tushar Sharma on-line! See this thread for details.
"Keeping the original header" isn't a reasonable requirement. The prolog is supposed to describe the document it is attached to, and that should be your requirement. So the encoding should declare the actual encoding of the document, and you should only have a "standalone" attribute in your prolog if the document is standalone.