Properly fix test failure with Rack 2.1+.

Rack is not to blame, just naive test case which was enough so far.

Fixes #1119

Gemfile

@@ -25,7 +25,6 @@ group :test, :development do
   gem 'multipart-parser'
   gem 'net-http-persistent', '~> 3.0'
   gem 'patron', '>= 0.4.2', platforms: :ruby
-  gem 'rack', '< 2.1'
   gem 'rack-test', '>= 0.6', require: 'rack/test'
   gem 'rspec', '~> 3.7'
   gem 'rspec_junit_formatter', '~> 0.4'

spec/support/shared_examples/request_method.rb

@@ -13,8 +13,8 @@
   end
 
   it 'handles headers with multiple values' do
-    request_stub.to_return(headers: { 'Set-Cookie' => 'one, two' })
-    expect(response.headers['set-cookie']).to eq('one, two')
+    request_stub.to_return(headers: { 'Set-Cookie' => 'name=value' })
+    expect(response.headers['set-cookie']).to eq('name=value')
   end
 
   it 'retrieves the response headers' do