Do not set Content-Type if params are explicitly set to nil

The breaking change was introduced with d016695
that disallows to send the payload with DELETE requests.

Make the request method skip setting default Content-Type header
if params are explicitly set to `nil`.

lib/rack/test.rb

@@ -217,31 +217,31 @@ def env_for(uri, env)
         # Stringifying and upcasing methods has be commit upstream
         env['REQUEST_METHOD'] ||= env[:method] ? env[:method].to_s.upcase : 'GET'
 
-        if %w[GET DELETE].include?(env['REQUEST_METHOD'])
+        params = env.delete(:params) { {} }
+
+        if env['REQUEST_METHOD'] == 'GET'
           # merge :params with the query string
-          if params = env[:params]
+          if params = params
             params = parse_nested_query(params) if params.is_a?(String)
 
             uri.query = [uri.query, build_nested_query(params)].compact.reject { |v| v == '' }.join('&')
           end
         elsif !env.key?(:input)
-          env['CONTENT_TYPE'] ||= 'application/x-www-form-urlencoded'
+          env['CONTENT_TYPE'] ||= 'application/x-www-form-urlencoded' unless params.nil?
 
-          if env[:params].is_a?(Hash)
-            if data = build_multipart(env[:params])
+          if params.is_a?(Hash)
+            if data = build_multipart(params)
               env[:input] = data
               env['CONTENT_LENGTH'] ||= data.length.to_s
               env['CONTENT_TYPE'] = "multipart/form-data; boundary=#{MULTIPART_BOUNDARY}"
             else
-              env[:input] = params_to_string(env[:params])
+              env[:input] = params_to_string(params)
             end
           else
-            env[:input] = env[:params]
+            env[:input] = params
           end
         end
 
-        env.delete(:params)
-
         set_cookie(env.delete(:cookie), uri) if env.key?(:cookie)
 
         Rack::MockRequest.env_for(uri.to_s, env)

spec/rack/test_spec.rb

@@ -463,6 +463,11 @@ def close
       expect(last_request.env['HTTP_USER_AGENT']).to eq('Rack::Test')
     end
 
+    it 'does not set CONTENT_TYPE if params are explicitly set to nil' do
+      public_send(verb, '/', nil)
+      expect(last_request.env['CONTENT_TYPE']).to be_nil
+    end
+
     it 'yields the response to a given block' do
       yielded = false
 
@@ -601,12 +606,6 @@ def verb
     def verb
       'delete'
     end
-
-    it 'does not set a content type' do
-      delete '/'
-
-      expect(last_request.env['CONTENT_TYPE']).to be_nil
-    end
   end
 
   describe '#options' do