Inconsistency with use of StaxDriver (XML includes header and is not pretty-printed)

As discussed with Jörg on the mailing list. Although the Javadoc and FAQ say that toXML will give pretty-printed XML 'snippets' (no header), this only occurs if not using the StaxDriver. There are 3 possible solution types:

a) Update Javadoc and FAQ to make clear that the header presence and pretty-printing is driver-dependent (and put info. about the header in the Javadoc).

b) Provide non-backwards-compatibility-breaking code to allow an 'XStream contract consistent' version of the StaX driver to be used (e.g., a separate driver class StaxContractConsistentDriver).

c) Fix the API 'properly' so that XStream always returns pretty-printed snippets, irrespective of the driver used. This might need to be a major release change since it could break code for users assuming the header existence when using StaxDriver.