Added Rack version specs to ensure the correct version is loaded.

spec/integration/rack_2_0/headers_spec.rb

@@ -1,6 +1,6 @@
 # frozen_string_literal: true
 
-describe Grape::Http::Headers, if: Gem::Version.new(Rack.release) < Gem::Version.new('3.0.0') do
+describe Grape::Http::Headers do
   it { expect(described_class::ALLOW).to eq('Allow') }
   it { expect(described_class::LOCATION).to eq('Location') }
   it { expect(described_class::TRANSFER_ENCODING).to eq('Transfer-Encoding') }

spec/integration/rack_2_0/version_spec.rb

@@ -0,0 +1,5 @@
+# frozen_string_literal: true
+
+describe Rack do
+  it { expect(Gem::Version.new(described_class.release).segments.first).to eq 2 }
+end

spec/integration/rack_3_0/headers_spec.rb

@@ -1,6 +1,6 @@
 # frozen_string_literal: true
 
-describe Grape::Http::Headers, if: Gem::Version.new(Rack.release) >= Gem::Version.new('3') do
+describe Grape::Http::Headers do
   it { expect(described_class::ALLOW).to eq('allow') }
   it { expect(described_class::LOCATION).to eq('location') }
   it { expect(described_class::TRANSFER_ENCODING).to eq('transfer-encoding') }

spec/integration/rack_3_0/version_spec.rb

@@ -0,0 +1,5 @@
+# frozen_string_literal: true
+
+describe Rack do
+  it { expect(Gem::Version.new(described_class.release).segments.first).to eq 3 }
+end