[csharp-netcore] Fix handling of Dictionary query parameters and fix deepObject style parameter handling

[Blackclaws]

This was triggered by this issue which it should close #8798

Looking into the issue I found that indeed SwaggerUI and the csharp-netcore client libraries were generating different queries. Neither of which was getting read by the newly updated aspnetcore generator (#8826). It appears that neither format was acceptable there and this is also something Swashbuckle needs to fix (domaindrivendev/Swashbuckle.AspNetCore#2034) as their generated spec does not actually work with the controller its supposed to query.

This pull request does two things:

• Fix default handling so that it corresponds to what SwaggerUI is doing for Dictionaries
• Fix deepObject handling so that it works for required parameters as well
• Fix deepObject handling for Dictionary style inline objects with no properties defined (this might not be completely to spec, but it makes sense to do so, see also: https://swagger.io/docs/specification/serialization/,
• Fix NullPointerException caused by the DefaultCodegen not finding a Schema

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  ./bin/utils/export_docs_generators.sh
  

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Closes #8798

[wing328]

cc @OpenAPITools/generator-core-team as well since the change covers DefaultCodegen

[wing328]

LGTM but I've not tested it locally

[wing328]

@Reinhard-PTV can you please test the test latest master to see if this change also works for your use case? Thank you.

[Reinhard-PTV]

@wing328 Tested with https://oss.sonatype.org/content/repositories/snapshots/org/openapitools/openapi-generator-cli/5.1.0-SNAPSHOT/openapi-generator-cli-5.1.0-20210304.083949-99.jar . Everything works fine.

[wing328]

@Reinhard-PTV thank you.

We've added a new library option httpclient so you may want to give it a try as well if you prefer it over restsharp.