This fixes Spyne to not crash on POST requests to the HTTP/RPC+JSON
webservice.

Note that all return values still use byte strings. Changing those will
touch a larger part of the codebase, and will be done in another commit.

As per [1]:

> 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.

[1] http://spyne.io/docs/2.10/manual/03_types.html#strings