headers['Content-Type'] & link_to_remote on Rails Edge

I'm on rails edge and am calling an after filter on my actions. Works except for my ajax calls via link_to_remote. When I inspect headers, I see the 'Content-Type' is key for an object rather than a string. Shouldn't it be a string? Anyone know if this is a bug or if I'm dealing with this incorrectly.

Thanks,

Jamie

The code:
   after_filter :set_charset
   def set_charset
     logger.debug("!!!!---- headers: #{headers.inspect}")
     unless headers['Content-Type'] =~ /charset/i
       headers['Content-Type'] ||= "text/html"
       headers['Content-Type'] += '; charset=utf-8'
     end
     headers['Pragma'] = 'no-cache'
     headers['Cache-Control'] = 'no-cache, no-store, must-revalidate'
   end

from the logger:
!!!!---- headers: {"Status"=>"200 OK", "cookie"=>[],
"Content-Type"=>#<Mime::Type:0x1316954
@synonyms=["application/javascript", "application/x-javascript"],
@symbol=:js, @string="text/javascript">,
"Cache-Control"=>"no-cache"}

Part of the stack trace:

NoMethodError (undefined method `+' for #<Mime::Type:0x1316954>):
     /app/controllers/application.rb:87:in `set_charset'
     .//vendor/rails/actionpack/lib/action_controller/filters.rb:471:in `call'
     .//vendor/rails/actionpack/lib/action_controller/filters.rb:465:in `call'
     .//vendor/rails/actionpack/lib/action_controller/filters.rb:633:in `call_filter'
     .//vendor/rails/actionpack/lib/action_controller/filters.rb:615:in `perform_action_without_benchmark'
     .//vendor/rails/actionpack/lib/action_controller/benchmarking.rb:66:in `perform_action_without_rescue'