You're sending the data as POST parameters, but the XBlock handler is expecting JSON.

When I try to submit feedback, I see this stack trace in the console:
```
Traceback (most recent call last):
  File "/Users/will/.virtualenvs/tim/lib/python2.7/site-packages/django/contrib/staticfiles/handlers.py", line 67, in __call__
    return self.application(environ, start_response)
  File "/Users/will/.virtualenvs/tim/lib/python2.7/site-packages/django/contrib/staticfiles/handlers.py", line 67, in __call__
    return self.application(environ, start_response)
  File "/Users/will/.virtualenvs/tim/lib/python2.7/site-packages/django/core/handlers/wsgi.py", line 241, in __call__
    response = self.get_response(request)
  File "/Users/will/.virtualenvs/tim/lib/python2.7/site-packages/django/core/handlers/base.py", line 179, in get_response
    response = self.handle_uncaught_exception(request, resolver, sys.exc_info())
  File "/Users/will/.virtualenvs/tim/lib/python2.7/site-packages/django/core/handlers/base.py", line 221, in handle_uncaught_exception
    return debug.technical_500_response(request, *exc_info)
  File "/Users/will/.virtualenvs/tim/lib/python2.7/site-packages/django_extensions/management/technical_response.py", line 5, in null_technical_500_response
    six.reraise(exc_type, exc_value, tb)
  File "/Users/will/.virtualenvs/tim/lib/python2.7/site-packages/django/core/handlers/base.py", line 111, in get_response
    response = callback(request, *callback_args, **callback_kwargs)
  File "/Users/will/.virtualenvs/tim/lib/python2.7/site-packages/workbench/views.py", line 114, in handler
    result = block.runtime.handle(block, handler_slug, request, suffix)
  File "/Users/will/.virtualenvs/tim/lib/python2.7/site-packages/xblock/runtime.py", line 643, in handle
    results = handler(request, suffix)
  File "/Users/will/.virtualenvs/tim/lib/python2.7/site-packages/xblock/core.py", line 114, in wrapper
    request_json = json.loads(request.body)
  File "/usr/local/Cellar/python/2.7.5/Frameworks/Python.framework/Versions/2.7/lib/python2.7/json/__init__.py", line 338, in loads
    return _default_decoder.decode(s)
  File "/usr/local/Cellar/python/2.7.5/Frameworks/Python.framework/Versions/2.7/lib/python2.7/json/decoder.py", line 365, in decode
    obj, end = self.raw_decode(s, idx=_w(s, 0).end())
  File "/usr/local/Cellar/python/2.7.5/Frameworks/Python.framework/Versions/2.7/lib/python2.7/json/decoder.py", line 383, in raw_decode
    raise ValueError("No JSON object could be decoded")
ValueError: No JSON object could be decoded
```