Adds the magic `frozen_string_literal: true` comment to every file and
enables a Rubocop rule to make sure that it's always going to be there
going forward as well.
See here for more background [1], but the basic idea is that unlike many
other languages, static strings in code are mutable by default. This has
since been acknowledged as not a particularly good idea, and the
intention is to rectify the mistake when Ruby 3 comes out, where all
string literals will be frozen. The `frozen_string_literal` magic
comment was introduced in Ruby 2.3 as a way of easing the transition,
and allows libraries and projects to freeze their literals in advance.
I don't think this is breaking in any way: it's possible that users
might've been pulling out one of are literals somehow and mutating it,
but that would probably not have been useful for anything and would
certainly not be recommended, so I'm quite comfortable pushing this
change through as a minor version.
As discussed in #641.
[1] https://stackoverflow.com/a/37799399
A few weeks back a new error type `idempotency_error` was introduced in
the API. I put it in to respond to #503, but then forgot to add support
for it in this library. This patch introduces a new exception class that
represents it.
Right now we have every error class in a separate file which doesn't do
anyone a lot of good, especially given that most of them are just stubs
that derive directly from `StripeError`.
This patch pulls them all into one file and gives them some
documentation. It follows #487 and #488 as minor refactoring work.
