Fix a problem with the dropdown list 'Tab Text' in the plugin settings

This is a little fix around the 'tab Text' drop down list.
Basically, the drop down list was always empty. I didn't even have the 'Select one' option.
Looking at the examples on the following page, it seems that it is a good idea to use html_safe:
http://api.rubyonrails.org/classes/ActionView/Helpers/FormTagHelper.html#method-i-select_tag
And it appeared that it fixed the problem.

I am using:
Redmine 2.1.4.stable
Ruby 1.8.7 (x86_64-linux)
Rails 3.2.8