handle empty string input parameters and return values

Created originally on Bitbucket by cmcdaniel (Craig McDaniel)

Changesets for this Pull Request have not been imported, because it had been already declined on Bitbucket. Marked as closed by the import user.

This is an attempt to handle both empty strings and None in unicode parameters and unicode return values. It was prompted by an issue with both the restxml and soap protocol implementations converting empty strings and None to u'None'.

An empty XML element such as "" or "" is considered None at least as far as xml.etree.Element is concerned, while JSON distinguishes between and empty string and None (nil). So the test_settext_empty in test_restjson and test_soap behaves differently.

This leaves the wsme-soap tests broken. I was unable to make the similar change there because of difficultly understanding how xsi:nil is used there.