noosfero | metadata plugin breaks the "product management" page (#81)

[Joenio Costa]

New comment for Issue 81

https://gitlab.com/noosfero/noosfero/issues/81#note_1400581


Author: Joenio Costa

Same error with field **iso8601**:

```
Started GET "/myprofile/colivre/manage_products/new" for X at 2015-06-18 13:14:09 -0300
Processing by ManageProductsController#new as HTML
  Parameters: {"profile"=>"colivre"}
  Rendered manage_products/_categories_for_selection.html.erb (37.3ms)
  Rendered manage_products/new.html.erb within layouts/application-ng (55.2ms)
  Rendered layouts/_javascript.html.erb (1.6ms)
Completed 500 Internal Server Error in 847.6ms

ActionView::Template::Error (undefined method `iso8601' for nil:NilClass):
    21:     <%= yield :head %>
    22:     <%=
    23:       @plugins.dispatch(:head_ending).map do |content|
    24:         if content.respond_to?(:call) then instance_exec(&content).to_s.html_safe else content.to_s.html_safe end
    25:       end.join("\n")
    26:     %>
    27: 
  config/plugins/metadata/lib/ext/product.rb:20:in `block in <class:Product>'
  config/plugins/metadata/lib/metadata_plugin.rb:67:in `call'
  config/plugins/metadata/lib/metadata_plugin.rb:67:in `block (3 levels) in head_ending'
  config/plugins/metadata/lib/metadata_plugin.rb:65:in `each'
  config/plugins/metadata/lib/metadata_plugin.rb:65:in `block (2 levels) in head_ending'
  config/plugins/metadata/lib/metadata_plugin.rb:59:in `each'
  config/plugins/metadata/lib/metadata_plugin.rb:59:in `block in head_ending'
  app/views/layouts/application-ng.html.erb:24:in `instance_exec'
  app/views/layouts/application-ng.html.erb:24:in `block in _app_views_layouts_application_ng_html_erb___174682699_102728330'
  app/views/layouts/application-ng.html.erb:23:in `map'
  app/views/layouts/application-ng.html.erb:23:in `_app_views_layouts_application_ng_html_erb___174682699_102728330'
```

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