[Swift4] bug fix for date encoding Issues

Created by: james-rantmedia

Description of the PR

Ensure the same date format string is used throughout the generated code (use the one set in Configuration.swift).

Ensure the same date formatter options are used when encoding dates as well as decoding dates. If a consumer has set their own date formatter on CodableHelper, use that when encoding dates too.

Adds DateFormatTests to the SWIFT4 unit tests.

Updates the SWIFT4 petstore samples

Fixes issue #1424 (closed)

PR checklist

• Read the contribution guidelines.
• Ran the shell 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). Windows batch files can be found in .\bin\windows\.
• Filed the PR against the correct branch: master, 3.4.x, 4.0.x. Default: master.
• Copied the technical committee to review the pull request if your PR is targeting a particular programming language.

cc @jgavris (2017/07) @ehyche (2017/08) @Edubits (2017/09) @jaz-ah (2017/09) @d-date (2018/03) for review