Bundler does not really have a concept of "development dependencies",
like RubyGems has. Bundler has the more generic concept of "groups".

Under the hood, the `gemspec` DSL will put gemspec development
dependencies under a `:development` Gemfile group, but there's no reason
to instantiate these as development dependencies, they are regular
runtime dependencies, except that they belong in a group named
:development.

By never instantiating development dependencies at all, we avoid having
to introduce hacks to "undo" the type Bundler does not know about, and I
also think the error messages read better.

https://github.com/rubygems/rubygems/commit/9a06fa5bda