Defines a
org.mule.api.transport.MessageAdapter that can have its payload re-assigned after it
has been created. Transport message adapters must never use this interface since a message payload received
from a transport is always consdered read-only.
Update the message payload. This is typically only called if the
payload was originally an InputStream. In which case, if the InputStream
is consumed, it needs to be replaced for future access.
Parameters: payload - the object to assign as the message payload