Depend on Rack ~> 3.1 instead of 2.2.

Author: gkellogg

rack-linkeddata.gemspec

@@ -29,7 +29,7 @@ Gem::Specification.new do |gem|
   gem.requirements               = []
   gem.add_runtime_dependency     'linkeddata', '~> 3.3'
   gem.add_runtime_dependency     'rack-rdf',   '~> 3.3'
-  gem.add_runtime_dependency     'rack',        '>= 2.2', '< 4'
+  gem.add_runtime_dependency     'rack',       '~> 3.1'
 
   gem.add_development_dependency 'yard' ,      '~> 0.9'
   gem.add_development_dependency 'rspec',      '~> 3.12'

spec/rack_spec.rb

@@ -99,6 +99,7 @@ def app
           expect(options).to include(:link)
           link = options[:link]
           expect(link).to eq %(<foo>; rel="self")
+          ''
         end
         get '/', {}, {"HTTP_ACCEPT" => 'application/n-triples', "HTTP_LINK" => %(<foo>; rel="self")}
       end
@@ -118,6 +119,7 @@ def app
             expect(options).to include(:accept_params)
             accept_params = options[:accept_params]
             expect(accept_params).to include(profile: "http://www.w3.org/ns/json-ld#compacted http://example.org/white-listed")
+            ''
           end
           get '/', {}, {"HTTP_ACCEPT" => header}
         end