I am calling a SOAP service that is using a different encoding than UTF-8 where is the option to set the encoding for the XML?

I have a set of SOAP services that I have to call as part of my normal processing.  What I am doing is converting the objects to xml using on output mapping, saving it and then posting the message.  The SOAP services will be changing soon from accepting UTF-8 to only accepting ISO-8859-1.   

What do I need to set, and where in order to have the encoding for all outgoing XML messages be ISO-8859-1?