[dart] Improve content-type handling

[kuhnroyal]

• fixes [BUG][dart-dio-next] Generator uses url encoded contentType even if the server content type requires application/json #9334
• superseeds [BUG][dart-dio-next] Fixes content-type prioritization #9454
• use prioritizedContentTypes in the same way JavaClientCodegen does
• move application/json to the front if it exists
• don't do anything if it is multi-part or url-encoded as for this the first content-type already needs to match
• log warning if an unsupported content-type is first after prioritizing
• remove some unused code blocks from dio generators

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CC @swipesight (2018/09) @jaumard (2018/09) @josh-burton (2019/12) @amondnet (2019/12) @sbu-WBT (2020/12) @kuhnroyal (2020/12) @agilob (2020/12)

[kuhnroyal]

@themisir Do you mind testing this for your use-case?

[themisir]

@themisir Do you mind testing this for your use-case?

Do I have a chance to test this without having to clone & re-building the project?

[kuhnroyal]

@themisir Do you mind testing this for your use-case?

Do I have a chance to test this without having to clone & re-building the project?

I don't think so, can't find the artifacts anywhere.

[themisir]

@themisir Do you mind testing this for your use-case?

Yes it works.

But why don't you directly generating code as contentType: 'value' instead of contentType: ['value'].first? If in the end, only the first value will be used on runtime, so what's the point of writing like [...].first? Maybe I'm missing something.

[kuhnroyal]

@themisir Do you mind testing this for your use-case?

Yes it works.

But why don't you directly generating code as contentType: 'value' instead of contentType: ['value'].first? If in the end, only the first value will be used on runtime, so what's the point of writing like [...].first? Maybe I'm missing something.

I am not sure this is possible in the template but I will check. In any case, they are now correctly ordered in a way that works.

[themisir]

I am not sure this is possible in the template but I will check

In my implementation, I introduced a vendorExtensions called x-content-type for that purpose.

But yeah, they are correctly ordered now.

[kuhnroyal]

@themisir I changed this to use only the first content-type.