Use `headers` instead of `header`, since in `Rack::Response` `headers` is an alias for `header` and `headers` is widely used in Sinatra. It's confusing that in `Sinatra::Response#finish` both `headers` and `header`.
Merge pull request #659 from patriciomacadden/header-headers

Use `headers` method instead of `header` method in `Sinatra::Response`