Improve rewindable input test coverage so tests fail when you remove the middleware

Signed-off-by: Joshua Peek <josh@joshpeek.com>

actionpack/test/dispatch/request/multipart_params_parsing_test.rb

@@ -206,8 +206,7 @@ def initialize(app)
       end
 
       def call(env)
-        req = Rack::Request.new(env)
-        req.params # Parse params
+        env['rack.input'].read
         @app.call(env)
       end
     end

actionpack/test/dispatch/request/url_encoded_params_parsing_test.rb

@@ -150,8 +150,7 @@ def initialize(app)
       end
 
       def call(env)
-        req = Rack::Request.new(env)
-        req.params # Parse params
+        env['rack.input'].read
         @app.call(env)
       end
     end