SkelView fails with some RelationalBones

[sveneberth]

The SkelView causes an error if in the skeleton is a

• non-multiple, not-multilang RelationalBone without an UsingSkel
• multiple, multilang RelationalBone without an UsingSkel
• multiple, multilang RelationalBone with an UsingSkel

(but I didn't tested all combinations).

Traceback (most recent call last):
  File "/.../deploy/viur/core/request.py", line 269, in processRequest
    self.findAndCall(path)
  File "/.../deploy/viur/core/request.py", line 608, in findAndCall
    res = caller(*newArgs, **newKwargs)
  File "/.../deploy/viur/core/prototypes/list.py", line 181, in list
    return self.render.list(res)
  File "/.../deploy/viur/core/render/html/default.py", line 432, in list
    return template.render(skellist=skellist, action=action, params=params, **kwargs)
  File "/.../site-packages/jinja2/environment.py", line 1291, in render
    self.environment.handle_exception()
  File "/.../site-packages/jinja2/environment.py", line 926, in handle_exception
    raise rewrite_traceback_stack(source=source)
  File "html/list.html", line 2, in top-level template code
    {% from "macros/skelview.html" import SkelView %}
  File "html/viur_base.html", line 119, in top-level template code
    {% block content %}
  File "html/list.html", line 12, in block 'content'
    {{ SkelView(skel) }}
  File "/.../site-packages/jinja2/runtime.py", line 777, in _invoke
    rv = self._func(*arguments)
  File "html/macros/skelview.html", line 298, in template
    {{ SkelViewBone(
  File "/.../site-packages/jinja2/runtime.py", line 777, in _invoke
    rv = self._func(*arguments)
  File "html/macros/skelview.html", line 234, in template
    {{ SkelViewRelBoneFormat(structure[step]["format"], item, prefix="dest.") }}
  File "/.../site-packages/jinja2/runtime.py", line 777, in _invoke
    rv = self._func(*arguments)
  File "html/macros/skelview.html", line 77, in template
    {% for key, val in value.items() %}
  File "/.../site-packages/jinja2/utils.py", line 83, in from_obj
    if hasattr(obj, "jinja_pass_arg"):
jinja2.exceptions.UndefinedError: 'str object' has no attribute 'items'