Add support for local timezone on ISO8601 output

[fetzi]

To be able to provide a appropriate time representation the fix
conversion to UTC is removed. time.Time holds the information about the timezone corresponding to the actual object and this timezone should be used for jsonapi representation.

Also the time.RFC3339 format is used instead of the custom format constant.

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[fetzi]

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[aren55555]

Given that the JSON API spec has a recommendation for using ISO 8601, I think we should try to stick to ISO 8601.

My simple understanding of ISO 8601 vs RFC 3339 is that they are very closely related however RFC 3339 is a simplification/profile of ISO 8601. I'm inclined to think that this library should aim to support ISO 8601 and that custom/wrapper golang types should be used to serialize RFC 3339 when needed by your use case.

It seems like what you are really after is the correct serialization/deserialization of the time zone information that was specified in the originating time value (whether that be in the JSON payload or a go variable of type time.Time). I think there should be a way to accomplish this without switching from ISO 8601 to RFC 3339.

[fetzi]

@aren55555 yeah, the main goal of this PR is to allow a correct serialization/deserialization of the timezone information.

According to the date formatting:
I've tried to modify the iso8601TimeFormat constant to allow the serialization/deserialization of the timezone information and the updated constant defines the exact same format as time.RFC3339. Therefore I didn't see the need for a custom constant any more.

But if you favour the custom constant, it is absolutely no problem to use an updated version of it:
iso8601TimeFormat = "2006-01-02T15:04:05Z07:00"

What do you think?

[fetzi]

@aren55555 any update on this PR?

[aren55555]

I'm pretty sure time.Time implements the json.Marshaler and json.Unmarshaler interfaces with time.RFC3339... meaning if you just remove all this it should just end up in that format.

[fetzi]

@aren55555 Parsing the time.Time values is done manually by calling time.Parse (needs layout information).

In the response part I tried to remove the Format call but then my added test case breaks. So I can confirm that the code in this PR is required to implement the expected behavior.

[penguinxr2]

I ran into the need for this as well, forcing times to UTC was unexpected.