Move test fixtures to more deep directory
Add testing frameworks to dependencies
Implement tests about extensions
Add note about serving zip files
[REFACTORING]Define TestZip#request and use it
[BUG FIX]Fix a bug about file extension

"/sample.zip".sub_ext('zip') results to "/sample.zip"
Add required Ruby version to gemspec
Add YARD to development dependencies
Add attribute reader "root"
Add more test data
Add a test case around multiple extensions
Add .gitignore
[REFACTORING]Extract calculation of clean path
[REFACTORING]find_zip_file -> file_zip_file_and_inner_path
[REFACTORING]Extract process to find file path with extension
[REFACTORING]Rewrite Rack::Zip#find_existing_file_with_extension using methods of Enumerable