Support for explicit 'base64 data types, empty lists, lists of strings, ...

...and empty strings.

Hi, this is my first pull request on GitHub, so please excuse any mistakes or unknown etiquette.  I have been maintaining my own xml-rpc.el improvements over 1.6.8 (from launchpad) since 2010 but am now moving my .el code that depends on xml-rpc.el to MELPA.  There, I saw that mentor (Stefan Kangas) uses your version 1.6.9 of xml-rpc.el so I am proposing to fold in my changes and bumping the version number on your end.  Then, I could stop maintaining my own fork/version.  I am impartial to whether calling it 1.6.9.1 or 1.6.10, so please feel free to change things around.

Hope my changes make sense.  If you prefer a finer grained history of my changes (they happened in 3 or 4 increments), I would be happy to sort those out and rearrange my git repo.  Or happy to answer any questions.

Here is a list of things that my new version does:

- Fixed bugs to be able to use empty lists and lists of lists of strings as XML parameters.
- Fixed bug that empty values were translated into a boolean (nil) instead of an empty string "" when turning XML into an Emacs list.
- Support for explicitly passing 'base64 data types.