Feature "default values for input" doesn't work

Steps to reproduce:
1. Create file with wf definition, that contain 'input' keyword and provide some default value for it.
For example:
---
version: '2.0'
wf1:
  type: reverse
  input:
    - greeting: "Hi"
  tasks:
    addressee:
      action: std.echo output="John"
      publish:
        name: <% $.addressee %>
    goodbye:
      action: std.echo output="<% $.greeting %>, <% $.name %>"
      requires: [addressee]
2. Execute "mistral workflow-create <file.yaml>"

Observed result:
A server error occurred. Please contact the administrator.
Traceback (most recent call last):
  File "/usr/lib/python2.7/wsgiref/handlers.py", line 85, in run
    self.result = application(self.environ, self.start_response)
  File "/usr/local/lib/python2.7/dist-packages/keystonemiddleware/auth_token/__init__.py", line 639, in __call__
    return self._call_app(env, start_response)
  File "/usr/local/lib/python2.7/dist-packages/keystonemiddleware/auth_token/__init__.py", line 559, in _call_app
    return self._app(env, _fake_start_response)
  File "/usr/local/lib/python2.7/dist-packages/pecan/middleware/recursive.py", line 56, in __call__
    return self.application(environ, start_response)
  File "/usr/local/lib/python2.7/dist-packages/pecan/core.py", line 812, in __call__
    return super(Pecan, self).__call__(environ, start_response)
  File "/usr/local/lib/python2.7/dist-packages/pecan/core.py", line 661, in __call__
    self.invoke_controller(controller, args, kwargs, state)
  File "/usr/local/lib/python2.7/dist-packages/pecan/core.py", line 561, in invoke_controller
    result = controller(*args, **kwargs)
  File "/home/ubuntu/mistral/mistral/utils/rest_utils.py", line 48, in wrapped
    return func(*args, **kwargs)
  File "/home/ubuntu/mistral/mistral/api/controllers/v2/workflow.py", line 122, in put
    workflow_list = [Workflow.from_dict(wf) for wf in models_dicts]
  File "/home/ubuntu/mistral/mistral/api/controllers/v2/workflow.py", line 73, in from_dict
    setattr(e, 'input', ", ".join(input) if input else None)
TypeError: sequence item 0: expected string, dict found