Fix build on Rbx.

I don't understand why this happens, but it's not material for the spec:

```
     Failure/Error: get '/thing' do
     TypeError:
       Unable to clone/dup String class
     # kernel/common/string.rb:39:in `dup (clone)'
     # /Users/dblock/.rvm/gems/rbx-2.2.6/gems/activesupport-4.1.8/lib/active_support/core_ext/object/deep_dup.rb:14:in `deep_dup'
     # /Users/dblock/.rvm/gems/rbx-2.2.6/gems/activesupport-4.1.8/lib/active_support/core_ext/object/deep_dup.rb:43:in `deep_dup'
     # kernel/common/enumerable.rb:83:in `each_with_object'
     # kernel/common/hash.rb:342:in `each'
     # kernel/common/enumerable.rb:81:in `each_with_object'
     # /Users/dblock/.rvm/gems/rbx-2.2.6/gems/activesupport-4.1.8/lib/active_support/core_ext/object/deep_dup.rb:42:in `deep_dup'
     # /Users/dblock/.rvm/gems/rbx-2.2.6/gems/activesupport-4.1.8/lib/active_support/core_ext/object/deep_dup.rb:43:in `deep_dup'
     # kernel/common/enumerable.rb:83:in `each_with_object'
     # kernel/common/hash.rb:342:in `each'
     # kernel/common/enumerable.rb:81:in `each_with_object'
     # /Users/dblock/.rvm/gems/rbx-2.2.6/gems/activesupport-4.1.8/lib/active_support/core_ext/object/deep_dup.rb:42:in `deep_dup'
     # /Users/dblock/.rvm/gems/rbx-2.2.6/gems/activesupport-4.1.8/lib/active_support/core_ext/object/deep_dup.rb:43:in `deep_dup'
     # kernel/common/enumerable.rb:83:in `each_with_object'
     # kernel/common/hash.rb:342:in `each'
     # kernel/common/enumerable.rb:81:in `each_with_object'
     # /Users/dblock/.rvm/gems/rbx-2.2.6/gems/activesupport-4.1.8/lib/active_support/core_ext/object/deep_dup.rb:42:in `deep_dup'
     # /Users/dblock/.rvm/gems/rbx-2.2.6/gems/activesupport-4.1.8/lib/active_support/core_ext/object/deep_dup.rb:43:in `deep_dup'
     # kernel/common/enumerable.rb:83:in `each_with_object'
     # kernel/common/hash.rb:342:in `each'
     # kernel/common/enumerable.rb:81:in `each_with_object'
     # /Users/dblock/.rvm/gems/rbx-2.2.6/gems/activesupport-4.1.8/lib/active_support/core_ext/object/deep_dup.rb:42:in `deep_dup'
     # /Users/dblock/.rvm/gems/rbx-2.2.6/bundler/gems/grape-443d48721fa6/lib/grape/util/inheritable_values.rb:39:in `initialize_copy'
     # kernel/common/kernel.rb:313:in `initialize_clone'
     # kernel/common/type.rb:432:in `object_initialize_clone'
     # kernel/alpha.rb:234:in `clone'
     # /Users/dblock/.rvm/gems/rbx-2.2.6/bundler/gems/grape-443d48721fa6/lib/grape/util/inheritable_setting.rb:49:in `point_in_time_copy'
     # kernel/common/kernel.rb:769:in `tap'
     # /Users/dblock/.rvm/gems/rbx-2.2.6/bundler/gems/grape-443d48721fa6/lib/grape/util/inheritable_setting.rb:45:in `point_in_time_copy'
     # /Users/dblock/.rvm/gems/rbx-2.2.6/bundler/gems/grape-443d48721fa6/lib/grape/endpoint.rb:55:in `initialize'
     # kernel/alpha.rb:94:in `new'
     # /Users/dblock/.rvm/gems/rbx-2.2.6/bundler/gems/grape-443d48721fa6/lib/grape/dsl/routing.rb:113:in `route'
     # /Users/dblock/.rvm/gems/rbx-2.2.6/bundler/gems/grape-443d48721fa6/lib/grape/dsl/routing.rb:123:in `__script__'
     # ./spec/api_global_models_spec.rb:25:in `__script__'
     # kernel/common/eval.rb:203:in `module_eval'
     # kernel/common/module.rb:33:in `initialize'
     # kernel/common/class.rb:18:in `initialize'
     # kernel/alpha.rb:94:in `new'
     # ./spec/api_global_models_spec.rb:22:in `__script__'
     # kernel/common/hash.rb:356:in `fetch'
     # ./spec/api_global_models_spec.rb:43:in `app'
     # /Users/dblock/.rvm/gems/rbx-2.2.6/gems/rack-test-0.6.2/lib/rack/test/methods.rb:31:in `build_rack_mock_session'
     # /Users/dblock/.rvm/gems/rbx-2.2.6/gems/rack-test-0.6.2/lib/rack/test/methods.rb:27:in `rack_mock_session'
     # /Users/dblock/.rvm/gems/rbx-2.2.6/gems/rack-test-0.6.2/lib/rack/test/methods.rb:42:in `build_rack_test_session'
     # /Users/dblock/.rvm/gems/rbx-2.2.6/gems/rack-test-0.6.2/lib/rack/test/methods.rb:38:in `rack_test_session'
     # /Users/dblock/.rvm/gems/rbx-2.2.6/gems/rack-test-0.6.2/lib/rack/test/methods.rb:46:in `current_session'
     # ./spec/api_global_models_spec.rb:60:in `__script__'
     # kernel/common/eval.rb:101:in `instance_exec'
     # kernel/bootstrap/array.rb:87:in `map'
     # kernel/bootstrap/array.rb:87:in `map'
     # kernel/common/kernel.rb:447:in `load'
     # kernel/common/block_environment.rb:53:in `call_on_instance'
     # kernel/common/eval.rb:176:in `eval'
     # kernel/delta/code_loader.rb:66:in `load_script'
     # kernel/delta/code_loader.rb:152:in `load_script'
     # kernel/loader.rb:649:in `script'
     # kernel/loader.rb:831:in `main'

```