Assume an uploaded scenario file (JSON) is encoded in UTF-8.

When uploading a file, web browsers do not specify a charset in
Content-Type because it is usually unknown and you cannot assume it
matches the form encoding.  Here's a fragment of multipart headers
Firefox will send:

```
Content-Disposition: form-data; name="scenario_import[file]"; filename="example.json"
Content-Type: application/json
```

This causes the scenario import to fail with the following error because
of an uploaded blob being treated as ASCII-8BIT:

```
ActionView::Template::Error (incompatible character encodings: ASCII-8BIT and UTF-8):
    18:     <% if @scenario_import.step_one? %>
    19:       <%= render 'step_one', :f => f %>
    20:     <% elsif @scenario_import.step_two? %>
    21:       <%= render 'step_two', :f => f %>
    22:     <% end %>
    23:   <% end %>
    24:
  app/views/scenario_imports/new.html.erb:21:in `block in _app_views_scenario_imports_new_html_erb___4262128004129331542_17292895160'
  app/views/scenario_imports/new.html.erb:15:in `_app_views_scenario_imports_new_html_erb___4262128004129331542_17292895160'
  app/controllers/scenario_imports_controller.rb:17:in `create'
  config/initializers/silence_worker_status_logger.rb:5:in `call_with_silence_worker_status'
```
Merge pull request #587 from knu/scenario_import_force_utf8

Assume an uploaded scenario file (JSON) is encoded in UTF-8.