[Swift3] null enumerations should not cause failure

[wwwjfy]

PR checklist

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• 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)
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Description of the PR

As all other the fields can be null, I think an enumeration field can also be null. The users can decide whether that's a problem. This is to be consistent with previous behaviors.
This was introduced in #4547 (@philippelatulippe)

The real change is only one line, but I cannot help but fix format issues.

[philippelatulippe]

Good catch, if the field is null it will return a typeMismatch, which is incorrect for decodeOptional(). But will this also return nil in the case that the field in the data is of the wrong type? I think the best behaviour is to have type mismatches return an error and only null or missing fields return nil. Perhaps this needs to be debated? The pros are that errors can be caught early, but the parser is less permissive.

[wwwjfy]

Thanks for reviewing the code. You're right, it shouldn't return success if the type mismatched.
My intention is to return success when it's null, missing the case of mismatched type.
I'll commit a fix later.

[wing328]

cc @jaz-ah @jgavris

[wwwjfy]

I pushed a fix.

Another similar place (which is left intact) is https://github.com/swagger-api/swagger-codegen/pull/4762/files#diff-940535e66d162a893df844fd9b6b45ffL202
The function signature is
static func decodeOptional<T: RawRepresentable, U: AnyObject where T.RawValue == U>(clazz: T, source: AnyObject) -> Decoded<T?>

I don't know when this function will be called. I'm a beginner in Swift. Isn't this case already covered in the function without U: AnyObject?

[jaz-ah]

+1 thx for the contribution @wwwjfy