Pull from stripe-ruby-mock:origin/master

[quekshuy]

This pull request introduces improvements to promotion code filtering and makes minor updates to gem usage and loading. The main change is the addition of case-insensitive filtering for promotion codes when listing, along with corresponding test coverage. There are also updates to the gem installation instructions and the main library file.

Promotion code filtering:

• Added case-insensitive filtering by code when listing promotion codes in list_promotion_code, ensuring that searches match regardless of letter case.
• Added a test to verify listing and filtering promotion codes by code in a case-insensitive manner in promotion_code_examples.rb.

Gem usage and loading:

• Updated the gem installation instructions in README.md to remove the explicit :require => 'stripe_mock' option, simplifying usage.
• Added a require 'stripe_mock' statement to the main stripe-ruby-mock.rb file to ensure proper loading of the gem.