Rack::Test is an unspecified runtime dependency

[wmene]

The Rubydora::Datastream#behaves_like_io? method references the Rack::Test::UploadedFile class

def behaves_like_io?(obj)
      obj.is_a?(IO) || (defined?(Rack) && obj.is_a?(Rack::Test::UploadedFile))
end

https://github.com/projecthydra/rubydora/blob/master/lib/rubydora/datastream.rb#L405

However, the gemspec does not declare rack-test as a runtime dependency. I was using rubydora/active-fedora in a Sinatra webservice when I kept getting the error

NameError: uninitialized constant Rack::Test

because this dependency was missing

[jcoyne]

I don't think we want to require rack-testas a dependency, but we should check to see if Rack::Test is defined.