### Changes:
- Using `Dictionary.init(minimumCapacity:)` to allocate the necessary size initially instead of growing when adding data to it
- Defined `uppercase` in terms of `lowercase`
- Removed magic `13` and `26` numbers
Fixes#1685.
### Description
Swift's `lazy` and `static` property initialization is always thread-safe. However, `key` was only created as an empty dictionary, and the evaluation of `ROT13.string`, which was the one initializing that data, wasn't isolated.
By creating `ROT13.key` inline, this ensures that the initialization is only done once in a thread-safe way.
Turns out `Data(string.utf8)` is a much simpler way of converting `String` to `Data` that doesn't lead to an `Optional`.
UTF-8 can represent all valid Unicode code points, so this was always safe, but now we know at compile time.
The reason the old variant returned an `Optional` is because it accepts other encodings like `.ascii` which might not be able to represent everything in the `String`.
This simplifies several parts of the code that were dealing with an error that was impossible.
Follow up to #1429. For #695.
This was duplicated in all operations.
`HTTPClient` now only forwards `Result.success` if the request actually succeeded,
leaving each `NetworkOperation` to just parse the body (which will become part of `HTTPClient` too).
Other changes:
- Simplified logic for when to retry requests in `HTTPClient` by using a new `Result.asOptionalResult`
- Removed several response handlers that don't need to do anything anymore
- Removed several tests that were basically duplicated:
- The `finishable` checks are already tested as part of `ErrorResponse`.
- All the checks for the error details are also part of `ErrorResponse`.
- No longer need to have a test for each type of failed response since that's part of `HTTPClient` tests now.
NachoSoto