Fix to avoid invalid JSON output

[dagwieers]

This PR fixes a few issues:

• Do not return None on empty strings
• Do not return NaN or infinity in JSON (invalid)
• Do not strip the value (assume that if it is like this in XML, that's actually on purpose)

This fixes #23

PS Do we have a location for adding unit-tests so we can test whether XMLData works as designed ?

[sanand0]

@dagwieers - the returned object is a Python object. Serializing into JSON returns a valid JSON object. For example:

>>> json.dumps(xmljson.badgerfish.data(fromstring('<root x="nan"/>')))
'{"root": {"@x": NaN}}'
>>> json.dumps(xmljson.badgerfish.data(fromstring('<root x=""/>')))
'{"root": {"@x": null}}'

The results are valid JSON. I don't think this PR is required... unless I'm missing something?

[dagwieers]

@sanand0 NaN and infinity are not accepted by the JSON standard, but apparently the python json library setsallow_nan=True. See: https://stackoverflow.com/questions/6601812/sending-nan-in-json

But apart from trying to fix that, this PR fixes a few other things, like empty strings should not be converted to null IMO.