module ActionDispatch::TestProcess

def assigns(key = nil) 

def assigns(key = nil)
  assigns = {}.with_indifferent_access
  @controller.view_assigns.each { |k, v| assigns.regular_writer(k, v) }
  key.nil? ? assigns : assigns[key]
end

def cookies 

def cookies
  @request.cookie_jar
end

def fixture_file_upload(path, mime_type = nil, binary = false)

post :change_avatar, avatar: fixture_file_upload('files/spongebob.png', 'image/png', :binary)

This will not affect other platforms:
To upload binary files on Windows, pass :binary as the last parameter.

post :change_avatar, avatar: fixture_file_upload('files/spongebob.png', 'image/png')

Shortcut for Rack::Test::UploadedFile.new(File.join(ActionController::TestCase.fixture_path, path), type): 
def fixture_file_upload(path, mime_type = nil, binary = false)
  if self.class.respond_to?(:fixture_path) && self.class.fixture_path
    path = File.join(self.class.fixture_path, path)
  end
  Rack::Test::UploadedFile.new(path, mime_type, binary)
end

def flash 

def flash
  @request.flash
end

def redirect_to_url 

def redirect_to_url
  @response.redirect_url
end

def session 

def session
  @request.session
end