Problem

1. If you use Rails 3.0.12, you are stuck with Rack 1.2.5
2. Rack 1.2.5 blows up if env['CONTENT_TYPE'] == "" and you call Rack::Request#media_type (or any other method that calls it, such as the very common pattern Rack::Request.new(env).params) per a bug that was fixed in later versions
3. At the same time, Rack::Lint complains if you set env['CONTENT_TYPE'] to nil.

Possible solution

I think unicorn (accidentially? intentionally?) solves this by defaulting to text/html... but I could be wrong:

# https://github.com/defunkt/unicorn/blob/master/lib/unicorn/cgi_wrapper.rb#L95
@head[CONTENT_TYPE] ||= "text/html"

So puma could default to text/html (?).