Changed return value of boolean method from nil to boolean value

[Shigeyuki-fukuda]

Summary

• I made boolean method appropriate. 8ef8400
  ex: firefox?
• I also fixed offenses of rubocop. 1cac594

Before

To give an example, when you accessed a website by Chrome.
Boolean methods will return nil except for case of Chrome.

$ browser = Browser.new(request.env['HTTP_USER_AGENT'])
$ browser.firefox?
=> nil

After

$ browser = Browser.new(request.env['HTTP_USER_AGENT'])
$ browser.firefox?
=> false

[fnando]

This subject (adding !! everywhere) has been discussed before. I don't think it's worth the trouble, so if you really care about the type, add !! to your code.

As for the offenses fixes, I fixed upstream due to conflicts. Thanks a lot for the PR.