Converts a java.util.Map to XML, specifying an 'entry'
element with 'key' and 'value' children.
Note: 'key' and 'value' is not the name of the generated tag. The
children are serialized as normal elements and the implementation expects
them in the order 'key'/'value'.
Supports java.util.HashMap, java.util.Hashtable and
java.util.LinkedHashMap.