fix bug undefined method when update custom fields

Call show_datail by send_issue_updated_to_hipchat.

https://github.com/digitalnatives/redmine_hipchat_per_project/blob/99d4b479d30b9853611fa169481a9757b...

```
details = journal.details.map{|d| show_detail(d) rescue show_detail(d, :no_html) }.join("<br />")
```

Show_datail belongs IssuesHelper.
https://github.com/redmine/redmine/blob/2.3-stable/app/helpers/issues_helper.rb#L305

But, format_value belongs CustomFieldsHelper.

https://github.com/redmine/redmine/blob/2.3-stable/app/helpers/custom_fields_helper.rb#L116

hipchat_notifier is not included CustomFieldsHelper.
That's raise undefined method because can't find format_value method.

I revised this bugs. Want you to check. Thank you.