The LeafElementSerializer class implements ElementSerializer. It is used to
serialize leaf elements in the message definition. If an ElementDefinition
does not specify a format definition, but does specify a datatype, then it is
a leaf element. Basic flow of the serialize method: 1. Based on the datatype
of the ElementDefinition, format the data appropriately. Note that in the
first version, the only supported datatype will be String, and therefore, no
reformatting is necessary. 2. Write the formatted data to the OutputStream.