Passing a string to a nested type fails to json-decode

[Adam Harwell]

With a nested type like this:

```
class PoolPOST(BasePoolType):
    name = wtypes.wsattr(wtypes.StringType(max_length=255))
    session_persistence = wtypes.wsattr(SessionPersistencePOST)

class SessionPersistencePOST(types.BaseType):
    type = wtypes.wsattr(wtypes.Enum(str, *constants.SUPPORTED_SP_TYPES),
                         mandatory=True)
    cookie_name = wtypes.wsattr(wtypes.StringType(max_length=255),
                                default=None)
```

If you POST something like this:

```
{
    "name": "hello",
    "session_persistence": ""
}
```

It will explode when trying to decode the subobject as JSON:

```
Traceback (most recent call last):
  File "python3.5/site-packages/wsmeext/pecan.py", line 81, in callfunction
    pecan.request.body, pecan.request.content_type
  File "/usr/lib/python3.5/site-packages/wsme/rest/args.py", line 296, in get_args
    from_body = args_from_body(funcdef, body, mimetype)
  File "python3.5/site-packages/wsme/rest/args.py", line 242, in args_from_body
    body, datatypes, bodyarg=funcdef.body_type is not None
  File "python3.5/site-packages/wsme/rest/json.py", line 266, in parse
    kw = {argname: fromjson(datatypes[argname], jdata)}
  File "python3.5/site-packages/simplegeneric.py", line 63, in dispatch
    return f(*args, **kw)
  File "python3.5/site-packages/wsme/rest/json.py", line 148, in fromjson
    value[attrdef.name])
  File "python3.5/site-packages/simplegeneric.py", line 63, in dispatch
    return f(*args, **kw)
  File "python3.5/site-packages/wsme/rest/json.py", line 148, in fromjson
    value[attrdef.name])
  File "python3.5/site-packages/simplegeneric.py", line 63, in dispatch
    return f(*args, **kw)
  File "python3.5/site-packages/wsme/rest/json.py", line 139, in fromjson
    v_keys = set(value.keys())
AttributeError: 'str' object has no attribute 'keys'
```

[Adam Harwell (adam-harwell)]

To be clear, I wouldn't expect it to be able to parse this string, but I *would* expect it to fail validation, instead of causing a 500 response.