Validate HTTP response length while receiving (#6891)

to_s method of HTTP::Response keeps blocking while it receives the whole
content, no matter how it is big. This means it may waste time to receive
unacceptably large files. It may also consume memory and disk in the
process. This solves the inefficency by checking response length while
receiving.

spec/models/concerns/remotable_spec.rb

@@ -29,7 +29,10 @@ RSpec.describe Remotable do
 
   context 'Remotable module is included' do
     before do
-      class Foo; include Remotable; end
+      class Foo
+        include Remotable
+        remotable_attachment :hoge, 1.kilobyte
+      end
     end
 
     let(:attribute_name) { "#{hoge}_remote_url".to_sym }