Collection improvements / fixes

[ob-stripe]

r? @brandur-stripe @remi-stripe
cc @stripe/api-libraries

A bunch of improvements and fixes to the Collection model:

• renamed requestParams to filters, which conveys the intent better and is consistent with stripe-ruby (this is a breaking change, though I don't think many users use this)
• override offsetGet (magic method invoked when accessing elements via []) to output a helpful error message when using numeric indices (this is consistent with stripe-ruby)
• add emptyCollection() static constructor, and nextPage() and previousPage() helper methods. Simplify autoPagingIterator() by using nextPage() (again, this is consistent with stripe-ruby)
• fix the all/create/retrieve method. Previously they updated the filters on the instance, which was incorrect. Now all correctly sets the filters on the returned object (not the object on which the method is called), and create and retrieve don't set the filters.
  • There's a bit of duplicated code between the trait methods for all/create/retrieve and the methods in Collection, but unfortunately PHP does not have anything like Ruby's include vs. extend to add trait methods as static methods or instance methods.
• added a bunch of tests for the new stuff

[brandur-stripe]

WFM. Thanks for the great comments too!