Add param validator for allowed values

[vickychijwani]

It would be great if a list of allowed values could be specified for a parameter, something like:

params do
  optional :status, :type => Symbol, :values => [:not_started, :in_process, :done]
end
get 'tasks' do
  # list tasks, optionally filtered by status
end

Then GET /tasks?status=done should return 200 OK, and GET /tasks?status=foobar should return 400 Bad Request.

I'm willing to try to implement this, if others are interested in such a feature.

[dblock]

e7c44e0