[Swift3] add en_US_POSIX locale to date formatters

[julienfouilhe]

PR checklist

• Read the contribution guidelines.
• Ran the shell/batch script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh and ./bin/security/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates)
• Filed the PR against the correct branch: master for non-breaking changes and 2.3.0 branch for breaking (non-backward compatible) changes.

Description of the PR

The same thing has been done for the swift generator, it resolves the same issue, described there: #2159
#5008

The only file edited is this one: modules/swagger-codegen/src/main/resources/swift3/Models.mustache

[wing328]

@julienfouilhe thanks for the PR.

cc @Edubits @jaz-ah @jgavris for review.

[jgavris]

👍 possible / easy to isolate a unit test in the example suite?

[julienfouilhe]

@jgavris Where are they located? I don't see any swift tests.

[wing328]

@julienfouilhe you can find the test cases for Swift3 Petstore client under https://github.com/swagger-api/swagger-codegen/tree/master/samples/client/petstore/swift3

[julienfouilhe]

@wing328 @jgavris After giving it some thought I'm not sure how to test this as it depends on the user's device locale. But here is a link to Apple's recommendations: https://developer.apple.com/library/content/documentation/Cocoa/Conceptual/DataFormatting/Articles/dfDateFormatting10_4.html

If you're working with fixed-format dates, you should first set the locale of the date formatter to something appropriate for your fixed format. In most cases the best locale to choose is en_US_POSIX, a locale that's specifically designed to yield US English results regardless of both user and system preferences. en_US_POSIX is also invariant in time (if the US, at some point in the future, changes the way it formats dates, en_US will change to reflect the new behavior, but en_US_POSIX will not), and between platforms (en_US_POSIX works the same on iPhone OS as it does on OS X, and as it does on other platforms).

Once you’ve set en_US_POSIX as the locale of the date formatter, you can then set the date format string and the date formatter will behave consistently for all users.

[jgavris]

I've had success swizzling +[NSLocale currentLocale] in tests (not in this project though). You could maybe do one or two examples.

[wing328]

@jgavris @jaz-ah @Edubits shall we add the tests in a separate PR instead and merge this PR into master if the PR looks good and has been tested manually?

[jaz-ah]

i'm ok w/ that @wing328

[jgavris]

👍 as well, sorry for the delay