Use unicode strings in webservice interface definitions

This causes us problems, such as the one from 5849fd76a

As per the Spyne documentation:

Unlike the Python str, the Spyne String is not for arbitrary byte streams. You should not use it unless you are absolutely, positively sure that you need to deal with text data with an unknown encoding. In all other cases, you should just use the Unicode type. They actually look the same from outside, this distinction is made just to properly deal with the quirks surrounding Python-2’s unicode type.

Remember that you have the ByteArray and File types at your disposal when you need to deal with arbitrary byte streams.

The String type will be just an alias for Unicode once Spyne gets ported to Python 3. It might even be deprecated and removed in the future, so make sure you are using either Unicode or ByteArray in your interface definitions.