MessageAggregateErrorHandler should include xml header (with encoding defined)

To provide support for different encodings (Such as ISO-8859-1), all xml messages should include the xml header with the encoding specified. If not, UTF-8 seems to be default which causes problems with some psecial characters.
The org.apache.servicemix.components.validation.MessageAggregatingErrorHandler doesn't include the xml header, which it should. One way is to have the encoding configurable through the constructor. Another way is for the class to, somehow, figure this out.