Use RSpec::RequestDescriber

Refs:

- [RSpecでRequest Describer - Qiita](http://qiita.com/r7kamura/items/a1ab81561cb6accae558)
- [r7kamura/rspec-request_describer](https://github.com/r7kamura/rspec-request_describer)
Prefer double-quoted strings unless you need single quotes to avoid extra backslashes for escaping.