Fix Mime::Type warning for Rails > 3.2

When using zipstream with Rails 3.2 or above, a warning appears in the logs:

> /Users/alisdair/.rbenv/versions/1.9.3-p392/lib/ruby/gems/1.9.1/gems/actionpack-3.2.13/lib/action_dispatch/http/mime_type.rb:102: warning: already initialized constant ZIP

[As of Rails 3.2](https://github.com/rails/rails/commit/d73269ba53992d8a01e5721aad3d23bc2b11dc4f), the "application/zip" mime type is already registered.

The pull request checks for the existence of the mime type before registering, which fixes the warning and should retain compatibility with Rails 3.1.