ActiveResource 'head' request sends headers with a nil key

I've noticed that ActiveResource.exists? doesn't seem to be working correctly. Here's the backtrace I'm getting:

NoMethodError: You have a nil object when you didn't expect it!
The error occurred while evaluating nil.downcase
 from /usr/local/lib/ruby/1.8/net/http.rb:1261:in `key?'
 from /usr/local/lib/ruby/1.8/net/http.rb:1150:in `initialize_http_header'
 from /usr/local/lib/ruby/1.8/net/http.rb:1149:in `each'
 from /usr/local/lib/ruby/1.8/net/http.rb:1149:in `initialize_http_header'
 from /usr/local/lib/ruby/1.8/net/http.rb:1481:in `initialize'
 from /usr/local/lib/ruby/1.8/net/http.rb:1597:in `initialize'
 from /usr/local/lib/ruby/1.8/net/http.rb:810:in `new'
 from /usr/local/lib/ruby/1.8/net/http.rb:810:in `head'
 from /Users/elliot/Sites/localhost/htdocs/proj/work/store/vendor/rails/activeresource/lib/active_resource/connection.rb:152:in `send'
 from /Users/elliot/Sites/localhost/htdocs/proj/work/store/vendor/rails/activeresource/lib/active_resource/connection.rb:152:in `request'
 from /Users/elliot/Sites/localhost/htdocs/proj/work/store/vendor/rails/activesupport/lib/active_support/core_ext/benchmark.rb:8:in `realtime'
 from /Users/elliot/Sites/localhost/htdocs/proj/work/store/vendor/rails/activeresource/lib/active_resource/connection.rb:152:in `request'
 from /Users/elliot/Sites/localhost/htdocs/proj/work/store/vendor/rails/activeresource/lib/active_resource/connection.rb:140:in `head'
 from /Users/elliot/Sites/localhost/htdocs/proj/work/store/vendor/rails/activeresource/lib/active_resource/base.rb:554:in `exists?'

I believe this happens because when ARes does a HEAD request, it's sending a headers hash that contains a key which is nil (because it's trying to send a format header along but there's no header defined for HEAD requests in ARes::Connection::HTTP_FORMAT_HEADER_NAMES). I've attached a failing test case that pinpoints this behavior. Is this normal?